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How to Write a Literature Review | Guide, Examples, & Templates

Published on January 2, 2023 by Shona McCombes . Revised on September 11, 2023.

What is a literature review? A literature review is a survey of scholarly sources on a specific topic. It provides an overview of current knowledge, allowing you to identify relevant theories, methods, and gaps in the existing research that you can later apply to your paper, thesis, or dissertation topic .

There are five key steps to writing a literature review:

• Search for relevant literature
• Evaluate sources
• Identify themes, debates, and gaps
• Outline the structure
• Write your literature review

A good literature review doesn’t just summarize sources—it analyzes, synthesizes , and critically evaluates to give a clear picture of the state of knowledge on the subject.

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Table of contents

What is the purpose of a literature review, examples of literature reviews, step 1 – search for relevant literature, step 2 – evaluate and select sources, step 3 – identify themes, debates, and gaps, step 4 – outline your literature review’s structure, step 5 – write your literature review, free lecture slides, other interesting articles, frequently asked questions, introduction.

• Quick Run-through
• Step 1 & 2

When you write a thesis , dissertation , or research paper , you will likely have to conduct a literature review to situate your research within existing knowledge. The literature review gives you a chance to:

• Demonstrate your familiarity with the topic and its scholarly context
• Develop a theoretical framework and methodology for your research
• Position your work in relation to other researchers and theorists
• Show how your research addresses a gap or contributes to a debate
• Evaluate the current state of research and demonstrate your knowledge of the scholarly debates around your topic.

Writing literature reviews is a particularly important skill if you want to apply for graduate school or pursue a career in research. We’ve written a step-by-step guide that you can follow below.

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Writing literature reviews can be quite challenging! A good starting point could be to look at some examples, depending on what kind of literature review you’d like to write.

• Example literature review #1: “Why Do People Migrate? A Review of the Theoretical Literature” ( Theoretical literature review about the development of economic migration theory from the 1950s to today.)
• Example literature review #2: “Literature review as a research methodology: An overview and guidelines” ( Methodological literature review about interdisciplinary knowledge acquisition and production.)
• Example literature review #3: “The Use of Technology in English Language Learning: A Literature Review” ( Thematic literature review about the effects of technology on language acquisition.)
• Example literature review #4: “Learners’ Listening Comprehension Difficulties in English Language Learning: A Literature Review” ( Chronological literature review about how the concept of listening skills has changed over time.)

You can also check out our templates with literature review examples and sample outlines at the links below.

Download Word doc Download Google doc

Before you begin searching for literature, you need a clearly defined topic .

If you are writing the literature review section of a dissertation or research paper, you will search for literature related to your research problem and questions .

Make a list of keywords

Start by creating a list of keywords related to your research question. Include each of the key concepts or variables you’re interested in, and list any synonyms and related terms. You can add to this list as you discover new keywords in the process of your literature search.

• Social media, Facebook, Instagram, Twitter, Snapchat, TikTok
• Body image, self-perception, self-esteem, mental health
• Generation Z, teenagers, adolescents, youth

Search for relevant sources

Use your keywords to begin searching for sources. Some useful databases to search for journals and articles include:

• Your university’s library catalogue
• Google Scholar
• Project Muse (humanities and social sciences)
• Medline (life sciences and biomedicine)
• EconLit (economics)
• Inspec (physics, engineering and computer science)

You can also use boolean operators to help narrow down your search.

Make sure to read the abstract to find out whether an article is relevant to your question. When you find a useful book or article, you can check the bibliography to find other relevant sources.

You likely won’t be able to read absolutely everything that has been written on your topic, so it will be necessary to evaluate which sources are most relevant to your research question.

For each publication, ask yourself:

• What question or problem is the author addressing?
• What are the key concepts and how are they defined?
• What are the key theories, models, and methods?
• Does the research use established frameworks or take an innovative approach?
• What are the results and conclusions of the study?
• How does the publication relate to other literature in the field? Does it confirm, add to, or challenge established knowledge?
• What are the strengths and weaknesses of the research?

Make sure the sources you use are credible , and make sure you read any landmark studies and major theories in your field of research.

You can use our template to summarize and evaluate sources you’re thinking about using. Click on either button below to download.

Take notes and cite your sources

As you read, you should also begin the writing process. Take notes that you can later incorporate into the text of your literature review.

It is important to keep track of your sources with citations to avoid plagiarism . It can be helpful to make an annotated bibliography , where you compile full citation information and write a paragraph of summary and analysis for each source. This helps you remember what you read and saves time later in the process.

To begin organizing your literature review’s argument and structure, be sure you understand the connections and relationships between the sources you’ve read. Based on your reading and notes, you can look for:

• Trends and patterns (in theory, method or results): do certain approaches become more or less popular over time?
• Themes: what questions or concepts recur across the literature?
• Debates, conflicts and contradictions: where do sources disagree?
• Pivotal publications: are there any influential theories or studies that changed the direction of the field?
• Gaps: what is missing from the literature? Are there weaknesses that need to be addressed?

This step will help you work out the structure of your literature review and (if applicable) show how your own research will contribute to existing knowledge.

• Most research has focused on young women.
• There is an increasing interest in the visual aspects of social media.
• But there is still a lack of robust research on highly visual platforms like Instagram and Snapchat—this is a gap that you could address in your own research.

There are various approaches to organizing the body of a literature review. Depending on the length of your literature review, you can combine several of these strategies (for example, your overall structure might be thematic, but each theme is discussed chronologically).

Chronological

The simplest approach is to trace the development of the topic over time. However, if you choose this strategy, be careful to avoid simply listing and summarizing sources in order.

Try to analyze patterns, turning points and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred.

If you have found some recurring central themes, you can organize your literature review into subsections that address different aspects of the topic.

For example, if you are reviewing literature about inequalities in migrant health outcomes, key themes might include healthcare policy, language barriers, cultural attitudes, legal status, and economic access.

Methodological

If you draw your sources from different disciplines or fields that use a variety of research methods , you might want to compare the results and conclusions that emerge from different approaches. For example:

• Look at what results have emerged in qualitative versus quantitative research
• Discuss how the topic has been approached by empirical versus theoretical scholarship
• Divide the literature into sociological, historical, and cultural sources

Theoretical

A literature review is often the foundation for a theoretical framework . You can use it to discuss various theories, models, and definitions of key concepts.

You might argue for the relevance of a specific theoretical approach, or combine various theoretical concepts to create a framework for your research.

Like any other academic text , your literature review should have an introduction , a main body, and a conclusion . What you include in each depends on the objective of your literature review.

The introduction should clearly establish the focus and purpose of the literature review.

Depending on the length of your literature review, you might want to divide the body into subsections. You can use a subheading for each theme, time period, or methodological approach.

As you write, you can follow these tips:

• Summarize and synthesize: give an overview of the main points of each source and combine them into a coherent whole
• Analyze and interpret: don’t just paraphrase other researchers — add your own interpretations where possible, discussing the significance of findings in relation to the literature as a whole
• Critically evaluate: mention the strengths and weaknesses of your sources
• Write in well-structured paragraphs: use transition words and topic sentences to draw connections, comparisons and contrasts

In the conclusion, you should summarize the key findings you have taken from the literature and emphasize their significance.

When you’ve finished writing and revising your literature review, don’t forget to proofread thoroughly before submitting. Not a language expert? Check out Scribbr’s professional proofreading services !

This article has been adapted into lecture slides that you can use to teach your students about writing a literature review.

Scribbr slides are free to use, customize, and distribute for educational purposes.

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If you want to know more about the research process , methodology , research bias , or statistics , make sure to check out some of our other articles with explanations and examples.

• Sampling methods
• Simple random sampling
• Stratified sampling
• Cluster sampling
• Likert scales
• Reproducibility

 Statistics

• Null hypothesis
• Statistical power
• Probability distribution
• Effect size
• Poisson distribution

Research bias

• Optimism bias
• Cognitive bias
• Implicit bias
• Hawthorne effect
• Anchoring bias
• Explicit bias

A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a thesis, dissertation , or research paper , in order to situate your work in relation to existing knowledge.

There are several reasons to conduct a literature review at the beginning of a research project:

• To familiarize yourself with the current state of knowledge on your topic
• To ensure that you’re not just repeating what others have already done
• To identify gaps in knowledge and unresolved problems that your research can address
• To develop your theoretical framework and methodology
• To provide an overview of the key findings and debates on the topic

Writing the literature review shows your reader how your work relates to existing research and what new insights it will contribute.

The literature review usually comes near the beginning of your thesis or dissertation . After the introduction , it grounds your research in a scholarly field and leads directly to your theoretical framework or methodology .

A literature review is a survey of credible sources on a topic, often used in dissertations , theses, and research papers . Literature reviews give an overview of knowledge on a subject, helping you identify relevant theories and methods, as well as gaps in existing research. Literature reviews are set up similarly to other  academic texts , with an introduction , a main body, and a conclusion .

An  annotated bibliography is a list of  source references that has a short description (called an annotation ) for each of the sources. It is often assigned as part of the research process for a  paper .  

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What is a Literature Review? How to Write It (with Examples)

A literature review is a critical analysis and synthesis of existing research on a particular topic. It provides an overview of the current state of knowledge, identifies gaps, and highlights key findings in the literature. 1 The purpose of a literature review is to situate your own research within the context of existing scholarship, demonstrating your understanding of the topic and showing how your work contributes to the ongoing conversation in the field. Learning how to write a literature review is a critical tool for successful research. Your ability to summarize and synthesize prior research pertaining to a certain topic demonstrates your grasp on the topic of study, and assists in the learning process. 

Table of Contents

What is the purpose of literature review , a. habitat loss and species extinction: , b. range shifts and phenological changes: , c. ocean acidification and coral reefs: , d. adaptive strategies and conservation efforts: .

• Choose a Topic and Define the Research Question: 
• Decide on the Scope of Your Review: 
• Select Databases for Searches: 
• Conduct Searches and Keep Track: 
• Review the Literature: 
• Organize and Write Your Literature Review: 
• How to write a literature review faster with Paperpal? 

Frequently asked questions 

What is a literature review .

A well-conducted literature review demonstrates the researcher’s familiarity with the existing literature, establishes the context for their own research, and contributes to scholarly conversations on the topic. One of the purposes of a literature review is also to help researchers avoid duplicating previous work and ensure that their research is informed by and builds upon the existing body of knowledge.

A literature review serves several important purposes within academic and research contexts. Here are some key objectives and functions of a literature review: 2  

1. Contextualizing the Research Problem: The literature review provides a background and context for the research problem under investigation. It helps to situate the study within the existing body of knowledge. 

2. Identifying Gaps in Knowledge: By identifying gaps, contradictions, or areas requiring further research, the researcher can shape the research question and justify the significance of the study. This is crucial for ensuring that the new research contributes something novel to the field.

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3. Understanding Theoretical and Conceptual Frameworks: Literature reviews help researchers gain an understanding of the theoretical and conceptual frameworks used in previous studies. This aids in the development of a theoretical framework for the current research. 

4. Providing Methodological Insights: Another purpose of literature reviews is that it allows researchers to learn about the methodologies employed in previous studies. This can help in choosing appropriate research methods for the current study and avoiding pitfalls that others may have encountered. 

5. Establishing Credibility: A well-conducted literature review demonstrates the researcher’s familiarity with existing scholarship, establishing their credibility and expertise in the field. It also helps in building a solid foundation for the new research. 

6. Informing Hypotheses or Research Questions: The literature review guides the formulation of hypotheses or research questions by highlighting relevant findings and areas of uncertainty in existing literature. 

Literature review example 

Let’s delve deeper with a literature review example: Let’s say your literature review is about the impact of climate change on biodiversity. You might format your literature review into sections such as the effects of climate change on habitat loss and species extinction, phenological changes, and marine biodiversity. Each section would then summarize and analyze relevant studies in those areas, highlighting key findings and identifying gaps in the research. The review would conclude by emphasizing the need for further research on specific aspects of the relationship between climate change and biodiversity. The following literature review template provides a glimpse into the recommended literature review structure and content, demonstrating how research findings are organized around specific themes within a broader topic. 

Literature Review on Climate Change Impacts on Biodiversity:  

Climate change is a global phenomenon with far-reaching consequences, including significant impacts on biodiversity. This literature review synthesizes key findings from various studies: 

Climate change-induced alterations in temperature and precipitation patterns contribute to habitat loss, affecting numerous species (Thomas et al., 2004). The review discusses how these changes increase the risk of extinction, particularly for species with specific habitat requirements. 

Observations of range shifts and changes in the timing of biological events (phenology) are documented in response to changing climatic conditions (Parmesan & Yohe, 2003). These shifts affect ecosystems and may lead to mismatches between species and their resources. 

The review explores the impact of climate change on marine biodiversity, emphasizing ocean acidification’s threat to coral reefs (Hoegh-Guldberg et al., 2007). Changes in pH levels negatively affect coral calcification, disrupting the delicate balance of marine ecosystems. 

Recognizing the urgency of the situation, the literature review discusses various adaptive strategies adopted by species and conservation efforts aimed at mitigating the impacts of climate change on biodiversity (Hannah et al., 2007). It emphasizes the importance of interdisciplinary approaches for effective conservation planning. 

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How to write a good literature review 

Writing a literature review involves summarizing and synthesizing existing research on a particular topic. A good literature review format should include the following elements. 

Introduction: The introduction sets the stage for your literature review, providing context and introducing the main focus of your review. 

• Opening Statement: Begin with a general statement about the broader topic and its significance in the field. 
• Scope and Purpose: Clearly define the scope of your literature review. Explain the specific research question or objective you aim to address. 
• Organizational Framework: Briefly outline the structure of your literature review, indicating how you will categorize and discuss the existing research. 
• Significance of the Study: Highlight why your literature review is important and how it contributes to the understanding of the chosen topic. 
• Thesis Statement: Conclude the introduction with a concise thesis statement that outlines the main argument or perspective you will develop in the body of the literature review. 

Body: The body of the literature review is where you provide a comprehensive analysis of existing literature, grouping studies based on themes, methodologies, or other relevant criteria. 

• Organize by Theme or Concept: Group studies that share common themes, concepts, or methodologies. Discuss each theme or concept in detail, summarizing key findings and identifying gaps or areas of disagreement. 
• Critical Analysis: Evaluate the strengths and weaknesses of each study. Discuss the methodologies used, the quality of evidence, and the overall contribution of each work to the understanding of the topic. 
• Synthesis of Findings: Synthesize the information from different studies to highlight trends, patterns, or areas of consensus in the literature. 
• Identification of Gaps: Discuss any gaps or limitations in the existing research and explain how your review contributes to filling these gaps. 
• Transition between Sections: Provide smooth transitions between different themes or concepts to maintain the flow of your literature review. 

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Conclusion: The conclusion of your literature review should summarize the main findings, highlight the contributions of the review, and suggest avenues for future research. 

• Summary of Key Findings: Recap the main findings from the literature and restate how they contribute to your research question or objective. 
• Contributions to the Field: Discuss the overall contribution of your literature review to the existing knowledge in the field. 
• Implications and Applications: Explore the practical implications of the findings and suggest how they might impact future research or practice. 
• Recommendations for Future Research: Identify areas that require further investigation and propose potential directions for future research in the field. 
• Final Thoughts: Conclude with a final reflection on the importance of your literature review and its relevance to the broader academic community. 

Conducting a literature review 

Conducting a literature review is an essential step in research that involves reviewing and analyzing existing literature on a specific topic. It’s important to know how to do a literature review effectively, so here are the steps to follow: 1  

Choose a Topic and Define the Research Question:  

• Select a topic that is relevant to your field of study. 
• Clearly define your research question or objective. Determine what specific aspect of the topic do you want to explore? 

Decide on the Scope of Your Review:  

• Determine the timeframe for your literature review. Are you focusing on recent developments, or do you want a historical overview? 
• Consider the geographical scope. Is your review global, or are you focusing on a specific region? 
• Define the inclusion and exclusion criteria. What types of sources will you include? Are there specific types of studies or publications you will exclude? 

Select Databases for Searches:  

• Identify relevant databases for your field. Examples include PubMed, IEEE Xplore, Scopus, Web of Science, and Google Scholar. 
• Consider searching in library catalogs, institutional repositories, and specialized databases related to your topic. 

Conduct Searches and Keep Track:  

• Develop a systematic search strategy using keywords, Boolean operators (AND, OR, NOT), and other search techniques. 
• Record and document your search strategy for transparency and replicability. 
• Keep track of the articles, including publication details, abstracts, and links. Use citation management tools like EndNote, Zotero, or Mendeley to organize your references. 

Review the Literature:  

• Evaluate the relevance and quality of each source. Consider the methodology, sample size, and results of studies. 
• Organize the literature by themes or key concepts. Identify patterns, trends, and gaps in the existing research. 
• Summarize key findings and arguments from each source. Compare and contrast different perspectives. 
• Identify areas where there is a consensus in the literature and where there are conflicting opinions. 
• Provide critical analysis and synthesis of the literature. What are the strengths and weaknesses of existing research? 

Organize and Write Your Literature Review:  

• Literature review outline should be based on themes, chronological order, or methodological approaches. 
• Write a clear and coherent narrative that synthesizes the information gathered. 
• Use proper citations for each source and ensure consistency in your citation style (APA, MLA, Chicago, etc.). 
• Conclude your literature review by summarizing key findings, identifying gaps, and suggesting areas for future research. 

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The literature review sample and detailed advice on writing and conducting a review will help you produce a well-structured report. But remember that a good literature review is an ongoing process, and it may be necessary to revisit and update it as your research progresses. By combining effortless research with an easy citation process, Paperpal Research streamlines the literature review process and empowers you to write faster and with more confidence. Try Paperpal Research now and see for yourself.  

A literature review is a critical and comprehensive analysis of existing literature (published and unpublished works) on a specific topic or research question and provides a synthesis of the current state of knowledge in a particular field. A well-conducted literature review is crucial for researchers to build upon existing knowledge, avoid duplication of efforts, and contribute to the advancement of their field. It also helps researchers situate their work within a broader context and facilitates the development of a sound theoretical and conceptual framework for their studies.

Literature review is a crucial component of research writing, providing a solid background for a research paper’s investigation. The aim is to keep professionals up to date by providing an understanding of ongoing developments within a specific field, including research methods, and experimental techniques used in that field, and present that knowledge in the form of a written report. Also, the depth and breadth of the literature review emphasizes the credibility of the scholar in his or her field.  

Before writing a literature review, it’s essential to undertake several preparatory steps to ensure that your review is well-researched, organized, and focused. This includes choosing a topic of general interest to you and doing exploratory research on that topic, writing an annotated bibliography, and noting major points, especially those that relate to the position you have taken on the topic. 

Literature reviews and academic research papers are essential components of scholarly work but serve different purposes within the academic realm. 3 A literature review aims to provide a foundation for understanding the current state of research on a particular topic, identify gaps or controversies, and lay the groundwork for future research. Therefore, it draws heavily from existing academic sources, including books, journal articles, and other scholarly publications. In contrast, an academic research paper aims to present new knowledge, contribute to the academic discourse, and advance the understanding of a specific research question. Therefore, it involves a mix of existing literature (in the introduction and literature review sections) and original data or findings obtained through research methods. 

Literature reviews are essential components of academic and research papers, and various strategies can be employed to conduct them effectively. If you want to know how to write a literature review for a research paper, here are four common approaches that are often used by researchers.  Chronological Review: This strategy involves organizing the literature based on the chronological order of publication. It helps to trace the development of a topic over time, showing how ideas, theories, and research have evolved.  Thematic Review: Thematic reviews focus on identifying and analyzing themes or topics that cut across different studies. Instead of organizing the literature chronologically, it is grouped by key themes or concepts, allowing for a comprehensive exploration of various aspects of the topic.  Methodological Review: This strategy involves organizing the literature based on the research methods employed in different studies. It helps to highlight the strengths and weaknesses of various methodologies and allows the reader to evaluate the reliability and validity of the research findings.  Theoretical Review: A theoretical review examines the literature based on the theoretical frameworks used in different studies. This approach helps to identify the key theories that have been applied to the topic and assess their contributions to the understanding of the subject.  It’s important to note that these strategies are not mutually exclusive, and a literature review may combine elements of more than one approach. The choice of strategy depends on the research question, the nature of the literature available, and the goals of the review. Additionally, other strategies, such as integrative reviews or systematic reviews, may be employed depending on the specific requirements of the research.

The literature review format can vary depending on the specific publication guidelines. However, there are some common elements and structures that are often followed. Here is a general guideline for the format of a literature review:  Introduction:   Provide an overview of the topic.  Define the scope and purpose of the literature review.  State the research question or objective.  Body:   Organize the literature by themes, concepts, or chronology.  Critically analyze and evaluate each source.  Discuss the strengths and weaknesses of the studies.  Highlight any methodological limitations or biases.  Identify patterns, connections, or contradictions in the existing research.  Conclusion:   Summarize the key points discussed in the literature review.  Highlight the research gap.  Address the research question or objective stated in the introduction.  Highlight the contributions of the review and suggest directions for future research.

Both annotated bibliographies and literature reviews involve the examination of scholarly sources. While annotated bibliographies focus on individual sources with brief annotations, literature reviews provide a more in-depth, integrated, and comprehensive analysis of existing literature on a specific topic. The key differences are as follows: 

References 

• Denney, A. S., & Tewksbury, R. (2013). How to write a literature review.  Journal of criminal justice education ,  24 (2), 218-234. 
• Pan, M. L. (2016).  Preparing literature reviews: Qualitative and quantitative approaches . Taylor & Francis. 
• Cantero, C. (2019). How to write a literature review.  San José State University Writing Center . 

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Writing a Literature Review

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A literature review is a document or section of a document that collects key sources on a topic and discusses those sources in conversation with each other (also called synthesis ). The lit review is an important genre in many disciplines, not just literature (i.e., the study of works of literature such as novels and plays). When we say “literature review” or refer to “the literature,” we are talking about the research ( scholarship ) in a given field. You will often see the terms “the research,” “the scholarship,” and “the literature” used mostly interchangeably.

Where, when, and why would I write a lit review?

There are a number of different situations where you might write a literature review, each with slightly different expectations; different disciplines, too, have field-specific expectations for what a literature review is and does. For instance, in the humanities, authors might include more overt argumentation and interpretation of source material in their literature reviews, whereas in the sciences, authors are more likely to report study designs and results in their literature reviews; these differences reflect these disciplines’ purposes and conventions in scholarship. You should always look at examples from your own discipline and talk to professors or mentors in your field to be sure you understand your discipline’s conventions, for literature reviews as well as for any other genre.

A literature review can be a part of a research paper or scholarly article, usually falling after the introduction and before the research methods sections. In these cases, the lit review just needs to cover scholarship that is important to the issue you are writing about; sometimes it will also cover key sources that informed your research methodology.

Lit reviews can also be standalone pieces, either as assignments in a class or as publications. In a class, a lit review may be assigned to help students familiarize themselves with a topic and with scholarship in their field, get an idea of the other researchers working on the topic they’re interested in, find gaps in existing research in order to propose new projects, and/or develop a theoretical framework and methodology for later research. As a publication, a lit review usually is meant to help make other scholars’ lives easier by collecting and summarizing, synthesizing, and analyzing existing research on a topic. This can be especially helpful for students or scholars getting into a new research area, or for directing an entire community of scholars toward questions that have not yet been answered.

What are the parts of a lit review?

Most lit reviews use a basic introduction-body-conclusion structure; if your lit review is part of a larger paper, the introduction and conclusion pieces may be just a few sentences while you focus most of your attention on the body. If your lit review is a standalone piece, the introduction and conclusion take up more space and give you a place to discuss your goals, research methods, and conclusions separately from where you discuss the literature itself.

Introduction:

• An introductory paragraph that explains what your working topic and thesis is
• A forecast of key topics or texts that will appear in the review
• Potentially, a description of how you found sources and how you analyzed them for inclusion and discussion in the review (more often found in published, standalone literature reviews than in lit review sections in an article or research paper)
• Summarize and synthesize: Give an overview of the main points of each source and combine them into a coherent whole
• Analyze and interpret: Don’t just paraphrase other researchers – add your own interpretations where possible, discussing the significance of findings in relation to the literature as a whole
• Critically Evaluate: Mention the strengths and weaknesses of your sources
• Write in well-structured paragraphs: Use transition words and topic sentence to draw connections, comparisons, and contrasts.

Conclusion:

• Summarize the key findings you have taken from the literature and emphasize their significance
• Connect it back to your primary research question

How should I organize my lit review?

Lit reviews can take many different organizational patterns depending on what you are trying to accomplish with the review. Here are some examples:

• Chronological : The simplest approach is to trace the development of the topic over time, which helps familiarize the audience with the topic (for instance if you are introducing something that is not commonly known in your field). If you choose this strategy, be careful to avoid simply listing and summarizing sources in order. Try to analyze the patterns, turning points, and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred (as mentioned previously, this may not be appropriate in your discipline — check with a teacher or mentor if you’re unsure).
• Thematic : If you have found some recurring central themes that you will continue working with throughout your piece, you can organize your literature review into subsections that address different aspects of the topic. For example, if you are reviewing literature about women and religion, key themes can include the role of women in churches and the religious attitude towards women.
• Qualitative versus quantitative research
• Empirical versus theoretical scholarship
• Divide the research by sociological, historical, or cultural sources
• Theoretical : In many humanities articles, the literature review is the foundation for the theoretical framework. You can use it to discuss various theories, models, and definitions of key concepts. You can argue for the relevance of a specific theoretical approach or combine various theorical concepts to create a framework for your research.

What are some strategies or tips I can use while writing my lit review?

Any lit review is only as good as the research it discusses; make sure your sources are well-chosen and your research is thorough. Don’t be afraid to do more research if you discover a new thread as you’re writing. More info on the research process is available in our "Conducting Research" resources .

As you’re doing your research, create an annotated bibliography ( see our page on the this type of document ). Much of the information used in an annotated bibliography can be used also in a literature review, so you’ll be not only partially drafting your lit review as you research, but also developing your sense of the larger conversation going on among scholars, professionals, and any other stakeholders in your topic.

Usually you will need to synthesize research rather than just summarizing it. This means drawing connections between sources to create a picture of the scholarly conversation on a topic over time. Many student writers struggle to synthesize because they feel they don’t have anything to add to the scholars they are citing; here are some strategies to help you:

• It often helps to remember that the point of these kinds of syntheses is to show your readers how you understand your research, to help them read the rest of your paper.
• Writing teachers often say synthesis is like hosting a dinner party: imagine all your sources are together in a room, discussing your topic. What are they saying to each other?
• Look at the in-text citations in each paragraph. Are you citing just one source for each paragraph? This usually indicates summary only. When you have multiple sources cited in a paragraph, you are more likely to be synthesizing them (not always, but often
• Read more about synthesis here.

The most interesting literature reviews are often written as arguments (again, as mentioned at the beginning of the page, this is discipline-specific and doesn’t work for all situations). Often, the literature review is where you can establish your research as filling a particular gap or as relevant in a particular way. You have some chance to do this in your introduction in an article, but the literature review section gives a more extended opportunity to establish the conversation in the way you would like your readers to see it. You can choose the intellectual lineage you would like to be part of and whose definitions matter most to your thinking (mostly humanities-specific, but this goes for sciences as well). In addressing these points, you argue for your place in the conversation, which tends to make the lit review more compelling than a simple reporting of other sources.

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A literature review is a review and synthesis of existing research on a topic or research question. A literature review is meant to analyze the scholarly literature, make connections across writings and identify strengths, weaknesses, trends, and missing conversations. A literature review should address different aspects of a topic as it relates to your research question. A literature review goes beyond a description or summary of the literature you have read. 

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• What is a Literature Review? | Guide, Template, & Examples

What is a Literature Review? | Guide, Template, & Examples

Published on 22 February 2022 by Shona McCombes . Revised on 7 June 2022.

What is a literature review? A literature review is a survey of scholarly sources on a specific topic. It provides an overview of current knowledge, allowing you to identify relevant theories, methods, and gaps in the existing research.

There are five key steps to writing a literature review:

• Search for relevant literature
• Evaluate sources
• Identify themes, debates and gaps
• Outline the structure
• Write your literature review

A good literature review doesn’t just summarise sources – it analyses, synthesises, and critically evaluates to give a clear picture of the state of knowledge on the subject.

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Table of contents

Why write a literature review, examples of literature reviews, step 1: search for relevant literature, step 2: evaluate and select sources, step 3: identify themes, debates and gaps, step 4: outline your literature review’s structure, step 5: write your literature review, frequently asked questions about literature reviews, introduction.

• Quick Run-through
• Step 1 & 2

When you write a dissertation or thesis, you will have to conduct a literature review to situate your research within existing knowledge. The literature review gives you a chance to:

• Demonstrate your familiarity with the topic and scholarly context
• Develop a theoretical framework and methodology for your research
• Position yourself in relation to other researchers and theorists
• Show how your dissertation addresses a gap or contributes to a debate

You might also have to write a literature review as a stand-alone assignment. In this case, the purpose is to evaluate the current state of research and demonstrate your knowledge of scholarly debates around a topic.

The content will look slightly different in each case, but the process of conducting a literature review follows the same steps. We’ve written a step-by-step guide that you can follow below.

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Writing literature reviews can be quite challenging! A good starting point could be to look at some examples, depending on what kind of literature review you’d like to write.

• Example literature review #1: “Why Do People Migrate? A Review of the Theoretical Literature” ( Theoretical literature review about the development of economic migration theory from the 1950s to today.)
• Example literature review #2: “Literature review as a research methodology: An overview and guidelines” ( Methodological literature review about interdisciplinary knowledge acquisition and production.)
• Example literature review #3: “The Use of Technology in English Language Learning: A Literature Review” ( Thematic literature review about the effects of technology on language acquisition.)
• Example literature review #4: “Learners’ Listening Comprehension Difficulties in English Language Learning: A Literature Review” ( Chronological literature review about how the concept of listening skills has changed over time.)

You can also check out our templates with literature review examples and sample outlines at the links below.

Download Word doc Download Google doc

Before you begin searching for literature, you need a clearly defined topic .

If you are writing the literature review section of a dissertation or research paper, you will search for literature related to your research objectives and questions .

If you are writing a literature review as a stand-alone assignment, you will have to choose a focus and develop a central question to direct your search. Unlike a dissertation research question, this question has to be answerable without collecting original data. You should be able to answer it based only on a review of existing publications.

Make a list of keywords

Start by creating a list of keywords related to your research topic. Include each of the key concepts or variables you’re interested in, and list any synonyms and related terms. You can add to this list if you discover new keywords in the process of your literature search.

• Social media, Facebook, Instagram, Twitter, Snapchat, TikTok
• Body image, self-perception, self-esteem, mental health
• Generation Z, teenagers, adolescents, youth

Search for relevant sources

Use your keywords to begin searching for sources. Some databases to search for journals and articles include:

• Your university’s library catalogue
• Google Scholar
• Project Muse (humanities and social sciences)
• Medline (life sciences and biomedicine)
• EconLit (economics)
• Inspec (physics, engineering and computer science)

You can use boolean operators to help narrow down your search:

Read the abstract to find out whether an article is relevant to your question. When you find a useful book or article, you can check the bibliography to find other relevant sources.

To identify the most important publications on your topic, take note of recurring citations. If the same authors, books or articles keep appearing in your reading, make sure to seek them out.

You probably won’t be able to read absolutely everything that has been written on the topic – you’ll have to evaluate which sources are most relevant to your questions.

For each publication, ask yourself:

• What question or problem is the author addressing?
• What are the key concepts and how are they defined?
• What are the key theories, models and methods? Does the research use established frameworks or take an innovative approach?
• What are the results and conclusions of the study?
• How does the publication relate to other literature in the field? Does it confirm, add to, or challenge established knowledge?
• How does the publication contribute to your understanding of the topic? What are its key insights and arguments?
• What are the strengths and weaknesses of the research?

Make sure the sources you use are credible, and make sure you read any landmark studies and major theories in your field of research.

You can find out how many times an article has been cited on Google Scholar – a high citation count means the article has been influential in the field, and should certainly be included in your literature review.

The scope of your review will depend on your topic and discipline: in the sciences you usually only review recent literature, but in the humanities you might take a long historical perspective (for example, to trace how a concept has changed in meaning over time).

Remember that you can use our template to summarise and evaluate sources you’re thinking about using!

Take notes and cite your sources

As you read, you should also begin the writing process. Take notes that you can later incorporate into the text of your literature review.

It’s important to keep track of your sources with references to avoid plagiarism . It can be helpful to make an annotated bibliography, where you compile full reference information and write a paragraph of summary and analysis for each source. This helps you remember what you read and saves time later in the process.

You can use our free APA Reference Generator for quick, correct, consistent citations.

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To begin organising your literature review’s argument and structure, you need to understand the connections and relationships between the sources you’ve read. Based on your reading and notes, you can look for:

• Trends and patterns (in theory, method or results): do certain approaches become more or less popular over time?
• Themes: what questions or concepts recur across the literature?
• Debates, conflicts and contradictions: where do sources disagree?
• Pivotal publications: are there any influential theories or studies that changed the direction of the field?
• Gaps: what is missing from the literature? Are there weaknesses that need to be addressed?

This step will help you work out the structure of your literature review and (if applicable) show how your own research will contribute to existing knowledge.

• Most research has focused on young women.
• There is an increasing interest in the visual aspects of social media.
• But there is still a lack of robust research on highly-visual platforms like Instagram and Snapchat – this is a gap that you could address in your own research.

There are various approaches to organising the body of a literature review. You should have a rough idea of your strategy before you start writing.

Depending on the length of your literature review, you can combine several of these strategies (for example, your overall structure might be thematic, but each theme is discussed chronologically).

Chronological

The simplest approach is to trace the development of the topic over time. However, if you choose this strategy, be careful to avoid simply listing and summarising sources in order.

Try to analyse patterns, turning points and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred.

If you have found some recurring central themes, you can organise your literature review into subsections that address different aspects of the topic.

For example, if you are reviewing literature about inequalities in migrant health outcomes, key themes might include healthcare policy, language barriers, cultural attitudes, legal status, and economic access.

Methodological

If you draw your sources from different disciplines or fields that use a variety of research methods , you might want to compare the results and conclusions that emerge from different approaches. For example:

• Look at what results have emerged in qualitative versus quantitative research
• Discuss how the topic has been approached by empirical versus theoretical scholarship
• Divide the literature into sociological, historical, and cultural sources

Theoretical

A literature review is often the foundation for a theoretical framework . You can use it to discuss various theories, models, and definitions of key concepts.

You might argue for the relevance of a specific theoretical approach, or combine various theoretical concepts to create a framework for your research.

Like any other academic text, your literature review should have an introduction , a main body, and a conclusion . What you include in each depends on the objective of your literature review.

The introduction should clearly establish the focus and purpose of the literature review.

If you are writing the literature review as part of your dissertation or thesis, reiterate your central problem or research question and give a brief summary of the scholarly context. You can emphasise the timeliness of the topic (“many recent studies have focused on the problem of x”) or highlight a gap in the literature (“while there has been much research on x, few researchers have taken y into consideration”).

Depending on the length of your literature review, you might want to divide the body into subsections. You can use a subheading for each theme, time period, or methodological approach.

As you write, make sure to follow these tips:

• Summarise and synthesise: give an overview of the main points of each source and combine them into a coherent whole.
• Analyse and interpret: don’t just paraphrase other researchers – add your own interpretations, discussing the significance of findings in relation to the literature as a whole.
• Critically evaluate: mention the strengths and weaknesses of your sources.
• Write in well-structured paragraphs: use transitions and topic sentences to draw connections, comparisons and contrasts.

In the conclusion, you should summarise the key findings you have taken from the literature and emphasise their significance.

If the literature review is part of your dissertation or thesis, reiterate how your research addresses gaps and contributes new knowledge, or discuss how you have drawn on existing theories and methods to build a framework for your research. This can lead directly into your methodology section.

A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a dissertation , thesis, research paper , or proposal .

There are several reasons to conduct a literature review at the beginning of a research project:

• To familiarise yourself with the current state of knowledge on your topic
• To ensure that you’re not just repeating what others have already done
• To identify gaps in knowledge and unresolved problems that your research can address
• To develop your theoretical framework and methodology
• To provide an overview of the key findings and debates on the topic

Writing the literature review shows your reader how your work relates to existing research and what new insights it will contribute.

The literature review usually comes near the beginning of your  dissertation . After the introduction , it grounds your research in a scholarly field and leads directly to your theoretical framework or methodology .

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What is a Literature Review?

A literature or narrative review is a comprehensive review and analysis of the published literature on a specific topic or research question. The literature that is reviewed contains: books, articles, academic articles, conference proceedings, association papers, and dissertations. It contains the most pertinent studies and points to important past and current research and practices. It provides background and context, and shows how your research will contribute to the field. 

A literature review should: 

• Provide a comprehensive and updated review of the literature;
• Explain why this review has taken place;
• Articulate a position or hypothesis;
• Acknowledge and account for conflicting and corroborating points of view

From  S age Research Methods

Purpose of a Literature Review

A literature review can be written as an introduction to a study to:

• Demonstrate how a study fills a gap in research
• Compare a study with other research that's been done

Or it can be a separate work (a research article on its own) which:

• Organizes or describes a topic
• Describes variables within a particular issue/problem

Limitations of a Literature Review

Some of the limitations of a literature review are:

• It's a snapshot in time. Unlike other reviews, this one has beginning, a middle and an end. There may be future developments that could make your work less relevant.
• It may be too focused. Some niche studies may miss the bigger picture.
• It can be difficult to be comprehensive. There is no way to make sure all the literature on a topic was considered.
• It is easy to be biased if you stick to top tier journals. There may be other places where people are publishing exemplary research. Look to open access publications and conferences to reflect a more inclusive collection. Also, make sure to include opposing views (and not just supporting evidence).

Source: Grant, Maria J., and Andrew Booth. “A Typology of Reviews: An Analysis of 14 Review Types and Associated Methodologies.” Health Information & Libraries Journal, vol. 26, no. 2, June 2009, pp. 91–108. Wiley Online Library, doi:10.1111/j.1471-1842.2009.00848.x.

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What is a literature review?

A literature review is an integrated analysis -- not just a summary-- of scholarly writings and other relevant evidence related directly to your research question.  That is, it represents a synthesis of the evidence that provides background information on your topic and shows a association between the evidence and your research question.

A literature review may be a stand alone work or the introduction to a larger research paper, depending on the assignment.  Rely heavily on the guidelines your instructor has given you.

Why is it important?

A literature review is important because it:

• Explains the background of research on a topic.
• Demonstrates why a topic is significant to a subject area.
• Discovers relationships between research studies/ideas.
• Identifies major themes, concepts, and researchers on a topic.
• Identifies critical gaps and points of disagreement.
• Discusses further research questions that logically come out of the previous studies.

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1. Choose a topic. Define your research question.

Your literature review should be guided by your central research question.  The literature represents background and research developments related to a specific research question, interpreted and analyzed by you in a synthesized way.

• Make sure your research question is not too broad or too narrow.  Is it manageable?
• Begin writing down terms that are related to your question. These will be useful for searches later.
• If you have the opportunity, discuss your topic with your professor and your class mates.

2. Decide on the scope of your review

How many studies do you need to look at? How comprehensive should it be? How many years should it cover? 

• This may depend on your assignment.  How many sources does the assignment require?

3. Select the databases you will use to conduct your searches.

Make a list of the databases you will search. 

Where to find databases:

• use the tabs on this guide
• Find other databases in the Nursing Information Resources web page
• More on the Medical Library web page
• ... and more on the Yale University Library web page

4. Conduct your searches to find the evidence. Keep track of your searches.

• Use the key words in your question, as well as synonyms for those words, as terms in your search. Use the database tutorials for help.
• Save the searches in the databases. This saves time when you want to redo, or modify, the searches. It is also helpful to use as a guide is the searches are not finding any useful results.
• Review the abstracts of research studies carefully. This will save you time.
• Use the bibliographies and references of research studies you find to locate others.
• Check with your professor, or a subject expert in the field, if you are missing any key works in the field.
• Ask your librarian for help at any time.
• Use a citation manager, such as EndNote as the repository for your citations. See the EndNote tutorials for help.

Review the literature

Some questions to help you analyze the research:

• What was the research question of the study you are reviewing? What were the authors trying to discover?
• Was the research funded by a source that could influence the findings?
• What were the research methodologies? Analyze its literature review, the samples and variables used, the results, and the conclusions.
• Does the research seem to be complete? Could it have been conducted more soundly? What further questions does it raise?
• If there are conflicting studies, why do you think that is?
• How are the authors viewed in the field? Has this study been cited? If so, how has it been analyzed?

Tips: 

• Review the abstracts carefully.  
• Keep careful notes so that you may track your thought processes during the research process.
• Create a matrix of the studies for easy analysis, and synthesis, across all of the studies.
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How to Write a Literature Review

What is a literature review.

• What Is the Literature
• Writing the Review

A literature review is much more than an annotated bibliography or a list of separate reviews of articles and books. It is a critical, analytical summary and synthesis of the current knowledge of a topic. Thus it should compare and relate different theories, findings, etc, rather than just summarize them individually. In addition, it should have a particular focus or theme to organize the review. It does not have to be an exhaustive account of everything published on the topic, but it should discuss all the significant academic literature and other relevant sources important for that focus.

This is meant to be a general guide to writing a literature review: ways to structure one, what to include, how it supplements other research. For more specific help on writing a review, and especially for help on finding the literature to review, sign up for a Personal Research Session .

The specific organization of a literature review depends on the type and purpose of the review, as well as on the specific field or topic being reviewed. But in general, it is a relatively brief but thorough exploration of past and current work on a topic. Rather than a chronological listing of previous work, though, literature reviews are usually organized thematically, such as different theoretical approaches, methodologies, or specific issues or concepts involved in the topic. A thematic organization makes it much easier to examine contrasting perspectives, theoretical approaches, methodologies, findings, etc, and to analyze the strengths and weaknesses of, and point out any gaps in, previous research. And this is the heart of what a literature review is about. A literature review may offer new interpretations, theoretical approaches, or other ideas; if it is part of a research proposal or report it should demonstrate the relationship of the proposed or reported research to others' work; but whatever else it does, it must provide a critical overview of the current state of research efforts. 

Literature reviews are common and very important in the sciences and social sciences. They are less common and have a less important role in the humanities, but they do have a place, especially stand-alone reviews.

Types of Literature Reviews

There are different types of literature reviews, and different purposes for writing a review, but the most common are:

• Stand-alone literature review articles . These provide an overview and analysis of the current state of research on a topic or question. The goal is to evaluate and compare previous research on a topic to provide an analysis of what is currently known, and also to reveal controversies, weaknesses, and gaps in current work, thus pointing to directions for future research. You can find examples published in any number of academic journals, but there is a series of Annual Reviews of *Subject* which are specifically devoted to literature review articles. Writing a stand-alone review is often an effective way to get a good handle on a topic and to develop ideas for your own research program. For example, contrasting theoretical approaches or conflicting interpretations of findings can be the basis of your research project: can you find evidence supporting one interpretation against another, or can you propose an alternative interpretation that overcomes their limitations?
• Part of a research proposal . This could be a proposal for a PhD dissertation, a senior thesis, or a class project. It could also be a submission for a grant. The literature review, by pointing out the current issues and questions concerning a topic, is a crucial part of demonstrating how your proposed research will contribute to the field, and thus of convincing your thesis committee to allow you to pursue the topic of your interest or a funding agency to pay for your research efforts.
• Part of a research report . When you finish your research and write your thesis or paper to present your findings, it should include a literature review to provide the context to which your work is a contribution. Your report, in addition to detailing the methods, results, etc. of your research, should show how your work relates to others' work.

A literature review for a research report is often a revision of the review for a research proposal, which can be a revision of a stand-alone review. Each revision should be a fairly extensive revision. With the increased knowledge of and experience in the topic as you proceed, your understanding of the topic will increase. Thus, you will be in a better position to analyze and critique the literature. In addition, your focus will change as you proceed in your research. Some areas of the literature you initially reviewed will be marginal or irrelevant for your eventual research, and you will need to explore other areas more thoroughly. 

Examples of Literature Reviews

See the series of Annual Reviews of *Subject* which are specifically devoted to literature review articles to find many examples of stand-alone literature reviews in the biomedical, physical, and social sciences. 

Research report articles vary in how they are organized, but a common general structure is to have sections such as:

• Abstract - Brief summary of the contents of the article
• Introduction - A explanation of the purpose of the study, a statement of the research question(s) the study intends to address
• Literature review - A critical assessment of the work done so far on this topic, to show how the current study relates to what has already been done
• Methods - How the study was carried out (e.g. instruments or equipment, procedures, methods to gather and analyze data)
• Results - What was found in the course of the study
• Discussion - What do the results mean
• Conclusion - State the conclusions and implications of the results, and discuss how it relates to the work reviewed in the literature review; also, point to directions for further work in the area

Here are some articles that illustrate variations on this theme. There is no need to read the entire articles (unless the contents interest you); just quickly browse through to see the sections, and see how each section is introduced and what is contained in them.

The Determinants of Undergraduate Grade Point Average: The Relative Importance of Family Background, High School Resources, and Peer Group Effects , in The Journal of Human Resources , v. 34 no. 2 (Spring 1999), p. 268-293.

This article has a standard breakdown of sections:

• Introduction
• Literature Review
• Some discussion sections

First Encounters of the Bureaucratic Kind: Early Freshman Experiences with a Campus Bureaucracy , in The Journal of Higher Education , v. 67 no. 6 (Nov-Dec 1996), p. 660-691.

This one does not have a section specifically labeled as a "literature review" or "review of the literature," but the first few sections cite a long list of other sources discussing previous research in the area before the authors present their own study they are reporting.

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What is a Literature Review?

So, what is a literature review .

"A literature review is an account of what has been published on a topic by accredited scholars and researchers. In writing the literature review, your purpose is to convey to your reader what knowledge and ideas have been established on a topic, and what their strengths and weaknesses are. As a piece of writing, the literature review must be defined by a guiding concept (e.g., your research objective, the problem or issue you are discussing, or your argumentative thesis). It is not just a descriptive list of the material available or a set of summaries." - Quote from Taylor, D. (n.d)."The Literature Review: A Few Tips on Conducting it".

• Citation: "The Literature Review: A Few Tips on Conducting it"

What kinds of literature reviews are written?

Each field has a particular way to do reviews for academic research literature. In the social sciences and humanities the most common are:

• Narrative Reviews: The purpose of this type of review is to describe the current state of the research on a specific research topic and to offer a critical analysis of the literature reviewed. Studies are grouped by research/theoretical categories, and themes and trends, strengths and weaknesses, and gaps are identified. The review ends with a conclusion section that summarizes the findings regarding the state of the research of the specific study, the gaps identify and if applicable, explains how the author's research will address gaps identify in the review and expand the knowledge on the topic reviewed.
• Book review essays/ Historiographical review essays : A type of literature review typical in History and related fields, e.g., Latin American studies. For example, the Latin American Research Review explains that the purpose of this type of review is to “(1) to familiarize readers with the subject, approach, arguments, and conclusions found in a group of books whose common focus is a historical period; a country or region within Latin America; or a practice, development, or issue of interest to specialists and others; (2) to locate these books within current scholarship, critical methodologies, and approaches; and (3) to probe the relation of these new books to previous work on the subject, especially canonical texts. Unlike individual book reviews, the cluster reviews found in LARR seek to address the state of the field or discipline and not solely the works at issue.” - LARR

What are the Goals of Creating a Literature Review?

• To develop a theory or evaluate an existing theory
• To summarize the historical or existing state of a research topic
• Identify a problem in a field of research 
• Baumeister, R.F. & Leary, M.R. (1997). "Writing narrative literature reviews," Review of General Psychology , 1(3), 311-320.

When do you need to write a Literature Review?

• When writing a prospectus or a thesis/dissertation
• When writing a research paper
• When writing a grant proposal

In all these cases you need to dedicate a chapter in these works to showcase what has been written about your research topic and to point out how your own research will shed new light into a body of scholarship.

Where I can find examples of Literature Reviews?

Note:  In the humanities, even if they don't use the term "literature review", they may have a dedicated  chapter that reviewed the "critical bibliography" or they incorporated that review in the introduction or first chapter of the dissertation, book, or article.

• UCSB electronic theses and dissertations In partnership with the Graduate Division, the UC Santa Barbara Library is making available theses and dissertations produced by UCSB students. Currently included in ADRL are theses and dissertations that were originally filed electronically, starting in 2011. In future phases of ADRL, all theses and dissertations created by UCSB students may be digitized and made available.

Where to Find Standalone Literature Reviews

Literature reviews are also written as standalone articles as a way to survey a particular research topic in-depth. This type of literature review looks at a topic from a historical perspective to see how the understanding of the topic has changed over time. 

• Find e-Journals for Standalone Literature Reviews The best way to get familiar with and to learn how to write literature reviews is by reading them. You can use our Journal Search option to find journals that specialize in publishing literature reviews from major disciplines like anthropology, sociology, etc. Usually these titles are called, "Annual Review of [discipline name] OR [Discipline name] Review. This option works best if you know the title of the publication you are looking for. Below are some examples of these journals! more... less... Journal Search can be found by hovering over the link for Research on the library website.

Social Sciences

• Annual Review of Anthropology
• Annual Review of Political Science
• Annual Review of Sociology
• Ethnic Studies Review

Hard science and health sciences:

• Annual Review of Biomedical Data Science
• Annual Review of Materials Science
• Systematic Review From journal site: "The journal Systematic Reviews encompasses all aspects of the design, conduct, and reporting of systematic reviews" in the health sciences.
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• Last Updated: Mar 5, 2024 11:44 AM
• URL: https://guides.library.ucsb.edu/litreview

What Is A Literature Review?

A plain-language explainer (with examples).

By:  Derek Jansen (MBA) & Kerryn Warren (PhD) | June 2020 (Updated May 2023)

If you’re faced with writing a dissertation or thesis, chances are you’ve encountered the term “literature review” . If you’re on this page, you’re probably not 100% what the literature review is all about. The good news is that you’ve come to the right place.

Literature Review 101

• What (exactly) is a literature review
• What’s the purpose of the literature review chapter
• How to find high-quality resources
• How to structure your literature review chapter
• Example of an actual literature review

What is a literature review?

The word “literature review” can refer to two related things that are part of the broader literature review process. The first is the task of  reviewing the literature  – i.e. sourcing and reading through the existing research relating to your research topic. The second is the  actual chapter  that you write up in your dissertation, thesis or research project. Let’s look at each of them:

Reviewing the literature

The first step of any literature review is to hunt down and  read through the existing research  that’s relevant to your research topic. To do this, you’ll use a combination of tools (we’ll discuss some of these later) to find journal articles, books, ebooks, research reports, dissertations, theses and any other credible sources of information that relate to your topic. You’ll then  summarise and catalogue these  for easy reference when you write up your literature review chapter. 

The literature review chapter

The second step of the literature review is to write the actual literature review chapter (this is usually the second chapter in a typical dissertation or thesis structure ). At the simplest level, the literature review chapter is an  overview of the key literature  that’s relevant to your research topic. This chapter should provide a smooth-flowing discussion of what research has already been done, what is known, what is unknown and what is contested in relation to your research topic. So, you can think of it as an  integrated review of the state of knowledge  around your research topic. 

What’s the purpose of a literature review?

The literature review chapter has a few important functions within your dissertation, thesis or research project. Let’s take a look at these:

Purpose #1 – Demonstrate your topic knowledge

The first function of the literature review chapter is, quite simply, to show the reader (or marker) that you  know what you’re talking about . In other words, a good literature review chapter demonstrates that you’ve read the relevant existing research and understand what’s going on – who’s said what, what’s agreed upon, disagreed upon and so on. This needs to be  more than just a summary  of who said what – it needs to integrate the existing research to  show how it all fits together  and what’s missing (which leads us to purpose #2, next). 

Purpose #2 – Reveal the research gap that you’ll fill

The second function of the literature review chapter is to  show what’s currently missing  from the existing research, to lay the foundation for your own research topic. In other words, your literature review chapter needs to show that there are currently “missing pieces” in terms of the bigger puzzle, and that  your study will fill one of those research gaps . By doing this, you are showing that your research topic is original and will help contribute to the body of knowledge. In other words, the literature review helps justify your research topic.  

Purpose #3 – Lay the foundation for your conceptual framework

The third function of the literature review is to form the  basis for a conceptual framework . Not every research topic will necessarily have a conceptual framework, but if your topic does require one, it needs to be rooted in your literature review. 

For example, let’s say your research aims to identify the drivers of a certain outcome – the factors which contribute to burnout in office workers. In this case, you’d likely develop a conceptual framework which details the potential factors (e.g. long hours, excessive stress, etc), as well as the outcome (burnout). Those factors would need to emerge from the literature review chapter – they can’t just come from your gut! 

So, in this case, the literature review chapter would uncover each of the potential factors (based on previous studies about burnout), which would then be modelled into a framework. 

Purpose #4 – To inform your methodology

The fourth function of the literature review is to  inform the choice of methodology  for your own research. As we’ve  discussed on the Grad Coach blog , your choice of methodology will be heavily influenced by your research aims, objectives and questions . Given that you’ll be reviewing studies covering a topic close to yours, it makes sense that you could learn a lot from their (well-considered) methodologies.

So, when you’re reviewing the literature, you’ll need to  pay close attention to the research design , methodology and methods used in similar studies, and use these to inform your methodology. Quite often, you’ll be able to  “borrow” from previous studies . This is especially true for quantitative studies , as you can use previously tried and tested measures and scales. 

How do I find articles for my literature review?

Finding quality journal articles is essential to crafting a rock-solid literature review. As you probably already know, not all research is created equally, and so you need to make sure that your literature review is  built on credible research . 

We could write an entire post on how to find quality literature (actually, we have ), but a good starting point is Google Scholar . Google Scholar is essentially the academic equivalent of Google, using Google’s powerful search capabilities to find relevant journal articles and reports. It certainly doesn’t cover every possible resource, but it’s a very useful way to get started on your literature review journey, as it will very quickly give you a good indication of what the  most popular pieces of research  are in your field.

One downside of Google Scholar is that it’s merely a search engine – that is, it lists the articles, but oftentimes  it doesn’t host the articles . So you’ll often hit a paywall when clicking through to journal websites. 

Thankfully, your university should provide you with access to their library, so you can find the article titles using Google Scholar and then search for them by name in your university’s online library. Your university may also provide you with access to  ResearchGate , which is another great source for existing research. 

Remember, the correct search keywords will be super important to get the right information from the start. So, pay close attention to the keywords used in the journal articles you read and use those keywords to search for more articles. If you can’t find a spoon in the kitchen, you haven’t looked in the right drawer. 

Need a helping hand?

How should I structure my literature review?

Unfortunately, there’s no generic universal answer for this one. The structure of your literature review will depend largely on your topic area and your research aims and objectives.

You could potentially structure your literature review chapter according to theme, group, variables , chronologically or per concepts in your field of research. We explain the main approaches to structuring your literature review here . You can also download a copy of our free literature review template to help you establish an initial structure.

In general, it’s also a good idea to start wide (i.e. the big-picture-level) and then narrow down, ending your literature review close to your research questions . However, there’s no universal one “right way” to structure your literature review. The most important thing is not to discuss your sources one after the other like a list – as we touched on earlier, your literature review needs to synthesise the research , not summarise it .

Ultimately, you need to craft your literature review so that it conveys the most important information effectively – it needs to tell a logical story in a digestible way. It’s no use starting off with highly technical terms and then only explaining what these terms mean later. Always assume your reader is not a subject matter expert and hold their hand through a journe y of the literature while keeping the functions of the literature review chapter (which we discussed earlier) front of mind.

Example of a literature review

In the video below, we walk you through a high-quality literature review from a dissertation that earned full distinction. This will give you a clearer view of what a strong literature review looks like in practice and hopefully provide some inspiration for your own. 

Wrapping Up

In this post, we’ve (hopefully) answered the question, “ what is a literature review? “. We’ve also considered the purpose and functions of the literature review, as well as how to find literature and how to structure the literature review chapter. If you’re keen to learn more, check out the literature review section of the Grad Coach blog , as well as our detailed video post covering how to write a literature review . 

Psst… there’s more!

This post is an extract from our bestselling short course, Literature Review Bootcamp . If you want to work smart, you don't want to miss this .

16 Comments

Thanks for this review. It narrates what’s not been taught as tutors are always in a early to finish their classes.

Thanks for the kind words, Becky. Good luck with your literature review 🙂

This website is amazing, it really helps break everything down. Thank you, I would have been lost without it.

This is review is amazing. I benefited from it a lot and hope others visiting this website will benefit too.

Timothy T. Chol [email protected]

Thank you very much for the guiding in literature review I learn and benefited a lot this make my journey smooth I’ll recommend this site to my friends

This was so useful. Thank you so much.

Hi, Concept was explained nicely by both of you. Thanks a lot for sharing it. It will surely help research scholars to start their Research Journey.

The review is really helpful to me especially during this period of covid-19 pandemic when most universities in my country only offer online classes. Great stuff

Great Brief Explanation, thanks

So helpful to me as a student

GradCoach is a fantastic site with brilliant and modern minds behind it.. I spent weeks decoding the substantial academic Jargon and grounding my initial steps on the research process, which could be shortened to a couple of days through the Gradcoach. Thanks again!

This is an amazing talk. I paved way for myself as a researcher. Thank you GradCoach!

Well-presented overview of the literature!

This was brilliant. So clear. Thank you

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• USC Libraries
• Research Guides

Organizing Your Social Sciences Research Paper

• 5. The Literature Review
• Purpose of Guide
• Design Flaws to Avoid
• Independent and Dependent Variables
• Glossary of Research Terms
• Reading Research Effectively
• Narrowing a Topic Idea
• Broadening a Topic Idea
• Extending the Timeliness of a Topic Idea
• Academic Writing Style
• Applying Critical Thinking
• Choosing a Title
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• The Research Problem/Question
• Theoretical Framework
• Citation Tracking
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• Evaluating Sources
• Primary Sources
• Secondary Sources
• Tiertiary Sources
• Scholarly vs. Popular Publications
• Qualitative Methods
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A literature review surveys prior research published in books, scholarly articles, and any other sources relevant to a particular issue, area of research, or theory, and by so doing, provides a description, summary, and critical evaluation of these works in relation to the research problem being investigated. Literature reviews are designed to provide an overview of sources you have used in researching a particular topic and to demonstrate to your readers how your research fits within existing scholarship about the topic.

Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . Fourth edition. Thousand Oaks, CA: SAGE, 2014.

Importance of a Good Literature Review

A literature review may consist of simply a summary of key sources, but in the social sciences, a literature review usually has an organizational pattern and combines both summary and synthesis, often within specific conceptual categories . A summary is a recap of the important information of the source, but a synthesis is a re-organization, or a reshuffling, of that information in a way that informs how you are planning to investigate a research problem. The analytical features of a literature review might:

• Give a new interpretation of old material or combine new with old interpretations,
• Trace the intellectual progression of the field, including major debates,
• Depending on the situation, evaluate the sources and advise the reader on the most pertinent or relevant research, or
• Usually in the conclusion of a literature review, identify where gaps exist in how a problem has been researched to date.

Given this, the purpose of a literature review is to:

• Place each work in the context of its contribution to understanding the research problem being studied.
• Describe the relationship of each work to the others under consideration.
• Identify new ways to interpret prior research.
• Reveal any gaps that exist in the literature.
• Resolve conflicts amongst seemingly contradictory previous studies.
• Identify areas of prior scholarship to prevent duplication of effort.
• Point the way in fulfilling a need for additional research.
• Locate your own research within the context of existing literature [very important].

Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper. 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Jesson, Jill. Doing Your Literature Review: Traditional and Systematic Techniques . Los Angeles, CA: SAGE, 2011; Knopf, Jeffrey W. "Doing a Literature Review." PS: Political Science and Politics 39 (January 2006): 127-132; Ridley, Diana. The Literature Review: A Step-by-Step Guide for Students . 2nd ed. Los Angeles, CA: SAGE, 2012.

Types of Literature Reviews

It is important to think of knowledge in a given field as consisting of three layers. First, there are the primary studies that researchers conduct and publish. Second are the reviews of those studies that summarize and offer new interpretations built from and often extending beyond the primary studies. Third, there are the perceptions, conclusions, opinion, and interpretations that are shared informally among scholars that become part of the body of epistemological traditions within the field.

In composing a literature review, it is important to note that it is often this third layer of knowledge that is cited as "true" even though it often has only a loose relationship to the primary studies and secondary literature reviews. Given this, while literature reviews are designed to provide an overview and synthesis of pertinent sources you have explored, there are a number of approaches you could adopt depending upon the type of analysis underpinning your study.

Argumentative Review This form examines literature selectively in order to support or refute an argument, deeply embedded assumption, or philosophical problem already established in the literature. The purpose is to develop a body of literature that establishes a contrarian viewpoint. Given the value-laden nature of some social science research [e.g., educational reform; immigration control], argumentative approaches to analyzing the literature can be a legitimate and important form of discourse. However, note that they can also introduce problems of bias when they are used to make summary claims of the sort found in systematic reviews [see below].

Integrative Review Considered a form of research that reviews, critiques, and synthesizes representative literature on a topic in an integrated way such that new frameworks and perspectives on the topic are generated. The body of literature includes all studies that address related or identical hypotheses or research problems. A well-done integrative review meets the same standards as primary research in regard to clarity, rigor, and replication. This is the most common form of review in the social sciences.

Historical Review Few things rest in isolation from historical precedent. Historical literature reviews focus on examining research throughout a period of time, often starting with the first time an issue, concept, theory, phenomena emerged in the literature, then tracing its evolution within the scholarship of a discipline. The purpose is to place research in a historical context to show familiarity with state-of-the-art developments and to identify the likely directions for future research.

Methodological Review A review does not always focus on what someone said [findings], but how they came about saying what they say [method of analysis]. Reviewing methods of analysis provides a framework of understanding at different levels [i.e. those of theory, substantive fields, research approaches, and data collection and analysis techniques], how researchers draw upon a wide variety of knowledge ranging from the conceptual level to practical documents for use in fieldwork in the areas of ontological and epistemological consideration, quantitative and qualitative integration, sampling, interviewing, data collection, and data analysis. This approach helps highlight ethical issues which you should be aware of and consider as you go through your own study.

Systematic Review This form consists of an overview of existing evidence pertinent to a clearly formulated research question, which uses pre-specified and standardized methods to identify and critically appraise relevant research, and to collect, report, and analyze data from the studies that are included in the review. The goal is to deliberately document, critically evaluate, and summarize scientifically all of the research about a clearly defined research problem . Typically it focuses on a very specific empirical question, often posed in a cause-and-effect form, such as "To what extent does A contribute to B?" This type of literature review is primarily applied to examining prior research studies in clinical medicine and allied health fields, but it is increasingly being used in the social sciences.

Theoretical Review The purpose of this form is to examine the corpus of theory that has accumulated in regard to an issue, concept, theory, phenomena. The theoretical literature review helps to establish what theories already exist, the relationships between them, to what degree the existing theories have been investigated, and to develop new hypotheses to be tested. Often this form is used to help establish a lack of appropriate theories or reveal that current theories are inadequate for explaining new or emerging research problems. The unit of analysis can focus on a theoretical concept or a whole theory or framework.

NOTE: Most often the literature review will incorporate some combination of types. For example, a review that examines literature supporting or refuting an argument, assumption, or philosophical problem related to the research problem will also need to include writing supported by sources that establish the history of these arguments in the literature.

Baumeister, Roy F. and Mark R. Leary. "Writing Narrative Literature Reviews."  Review of General Psychology 1 (September 1997): 311-320; Mark R. Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Kennedy, Mary M. "Defining a Literature." Educational Researcher 36 (April 2007): 139-147; Petticrew, Mark and Helen Roberts. Systematic Reviews in the Social Sciences: A Practical Guide . Malden, MA: Blackwell Publishers, 2006; Torracro, Richard. "Writing Integrative Literature Reviews: Guidelines and Examples." Human Resource Development Review 4 (September 2005): 356-367; Rocco, Tonette S. and Maria S. Plakhotnik. "Literature Reviews, Conceptual Frameworks, and Theoretical Frameworks: Terms, Functions, and Distinctions." Human Ressource Development Review 8 (March 2008): 120-130; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016.

Structure and Writing Style

I.  Thinking About Your Literature Review

The structure of a literature review should include the following in support of understanding the research problem :

• An overview of the subject, issue, or theory under consideration, along with the objectives of the literature review,
• Division of works under review into themes or categories [e.g. works that support a particular position, those against, and those offering alternative approaches entirely],
• An explanation of how each work is similar to and how it varies from the others,
• Conclusions as to which pieces are best considered in their argument, are most convincing of their opinions, and make the greatest contribution to the understanding and development of their area of research.

The critical evaluation of each work should consider :

• Provenance -- what are the author's credentials? Are the author's arguments supported by evidence [e.g. primary historical material, case studies, narratives, statistics, recent scientific findings]?
• Methodology -- were the techniques used to identify, gather, and analyze the data appropriate to addressing the research problem? Was the sample size appropriate? Were the results effectively interpreted and reported?
• Objectivity -- is the author's perspective even-handed or prejudicial? Is contrary data considered or is certain pertinent information ignored to prove the author's point?
• Persuasiveness -- which of the author's theses are most convincing or least convincing?
• Validity -- are the author's arguments and conclusions convincing? Does the work ultimately contribute in any significant way to an understanding of the subject?

II.  Development of the Literature Review

Four Basic Stages of Writing 1.  Problem formulation -- which topic or field is being examined and what are its component issues? 2.  Literature search -- finding materials relevant to the subject being explored. 3.  Data evaluation -- determining which literature makes a significant contribution to the understanding of the topic. 4.  Analysis and interpretation -- discussing the findings and conclusions of pertinent literature.

Consider the following issues before writing the literature review: Clarify If your assignment is not specific about what form your literature review should take, seek clarification from your professor by asking these questions: 1.  Roughly how many sources would be appropriate to include? 2.  What types of sources should I review (books, journal articles, websites; scholarly versus popular sources)? 3.  Should I summarize, synthesize, or critique sources by discussing a common theme or issue? 4.  Should I evaluate the sources in any way beyond evaluating how they relate to understanding the research problem? 5.  Should I provide subheadings and other background information, such as definitions and/or a history? Find Models Use the exercise of reviewing the literature to examine how authors in your discipline or area of interest have composed their literature review sections. Read them to get a sense of the types of themes you might want to look for in your own research or to identify ways to organize your final review. The bibliography or reference section of sources you've already read, such as required readings in the course syllabus, are also excellent entry points into your own research. Narrow the Topic The narrower your topic, the easier it will be to limit the number of sources you need to read in order to obtain a good survey of relevant resources. Your professor will probably not expect you to read everything that's available about the topic, but you'll make the act of reviewing easier if you first limit scope of the research problem. A good strategy is to begin by searching the USC Libraries Catalog for recent books about the topic and review the table of contents for chapters that focuses on specific issues. You can also review the indexes of books to find references to specific issues that can serve as the focus of your research. For example, a book surveying the history of the Israeli-Palestinian conflict may include a chapter on the role Egypt has played in mediating the conflict, or look in the index for the pages where Egypt is mentioned in the text. Consider Whether Your Sources are Current Some disciplines require that you use information that is as current as possible. This is particularly true in disciplines in medicine and the sciences where research conducted becomes obsolete very quickly as new discoveries are made. However, when writing a review in the social sciences, a survey of the history of the literature may be required. In other words, a complete understanding the research problem requires you to deliberately examine how knowledge and perspectives have changed over time. Sort through other current bibliographies or literature reviews in the field to get a sense of what your discipline expects. You can also use this method to explore what is considered by scholars to be a "hot topic" and what is not.

III.  Ways to Organize Your Literature Review

Chronology of Events If your review follows the chronological method, you could write about the materials according to when they were published. This approach should only be followed if a clear path of research building on previous research can be identified and that these trends follow a clear chronological order of development. For example, a literature review that focuses on continuing research about the emergence of German economic power after the fall of the Soviet Union. By Publication Order your sources by publication chronology, then, only if the order demonstrates a more important trend. For instance, you could order a review of literature on environmental studies of brown fields if the progression revealed, for example, a change in the soil collection practices of the researchers who wrote and/or conducted the studies. Thematic [“conceptual categories”] A thematic literature review is the most common approach to summarizing prior research in the social and behavioral sciences. Thematic reviews are organized around a topic or issue, rather than the progression of time, although the progression of time may still be incorporated into a thematic review. For example, a review of the Internet’s impact on American presidential politics could focus on the development of online political satire. While the study focuses on one topic, the Internet’s impact on American presidential politics, it would still be organized chronologically reflecting technological developments in media. The difference in this example between a "chronological" and a "thematic" approach is what is emphasized the most: themes related to the role of the Internet in presidential politics. Note that more authentic thematic reviews tend to break away from chronological order. A review organized in this manner would shift between time periods within each section according to the point being made. Methodological A methodological approach focuses on the methods utilized by the researcher. For the Internet in American presidential politics project, one methodological approach would be to look at cultural differences between the portrayal of American presidents on American, British, and French websites. Or the review might focus on the fundraising impact of the Internet on a particular political party. A methodological scope will influence either the types of documents in the review or the way in which these documents are discussed.

Other Sections of Your Literature Review Once you've decided on the organizational method for your literature review, the sections you need to include in the paper should be easy to figure out because they arise from your organizational strategy. In other words, a chronological review would have subsections for each vital time period; a thematic review would have subtopics based upon factors that relate to the theme or issue. However, sometimes you may need to add additional sections that are necessary for your study, but do not fit in the organizational strategy of the body. What other sections you include in the body is up to you. However, only include what is necessary for the reader to locate your study within the larger scholarship about the research problem.

Here are examples of other sections, usually in the form of a single paragraph, you may need to include depending on the type of review you write:

• Current Situation : Information necessary to understand the current topic or focus of the literature review.
• Sources Used : Describes the methods and resources [e.g., databases] you used to identify the literature you reviewed.
• History : The chronological progression of the field, the research literature, or an idea that is necessary to understand the literature review, if the body of the literature review is not already a chronology.
• Selection Methods : Criteria you used to select (and perhaps exclude) sources in your literature review. For instance, you might explain that your review includes only peer-reviewed [i.e., scholarly] sources.
• Standards : Description of the way in which you present your information.
• Questions for Further Research : What questions about the field has the review sparked? How will you further your research as a result of the review?

IV.  Writing Your Literature Review

Once you've settled on how to organize your literature review, you're ready to write each section. When writing your review, keep in mind these issues.

Use Evidence A literature review section is, in this sense, just like any other academic research paper. Your interpretation of the available sources must be backed up with evidence [citations] that demonstrates that what you are saying is valid. Be Selective Select only the most important points in each source to highlight in the review. The type of information you choose to mention should relate directly to the research problem, whether it is thematic, methodological, or chronological. Related items that provide additional information, but that are not key to understanding the research problem, can be included in a list of further readings . Use Quotes Sparingly Some short quotes are appropriate if you want to emphasize a point, or if what an author stated cannot be easily paraphrased. Sometimes you may need to quote certain terminology that was coined by the author, is not common knowledge, or taken directly from the study. Do not use extensive quotes as a substitute for using your own words in reviewing the literature. Summarize and Synthesize Remember to summarize and synthesize your sources within each thematic paragraph as well as throughout the review. Recapitulate important features of a research study, but then synthesize it by rephrasing the study's significance and relating it to your own work and the work of others. Keep Your Own Voice While the literature review presents others' ideas, your voice [the writer's] should remain front and center. For example, weave references to other sources into what you are writing but maintain your own voice by starting and ending the paragraph with your own ideas and wording. Use Caution When Paraphrasing When paraphrasing a source that is not your own, be sure to represent the author's information or opinions accurately and in your own words. Even when paraphrasing an author’s work, you still must provide a citation to that work.

V.  Common Mistakes to Avoid

These are the most common mistakes made in reviewing social science research literature.

• Sources in your literature review do not clearly relate to the research problem;
• You do not take sufficient time to define and identify the most relevant sources to use in the literature review related to the research problem;
• Relies exclusively on secondary analytical sources rather than including relevant primary research studies or data;
• Uncritically accepts another researcher's findings and interpretations as valid, rather than examining critically all aspects of the research design and analysis;
• Does not describe the search procedures that were used in identifying the literature to review;
• Reports isolated statistical results rather than synthesizing them in chi-squared or meta-analytic methods; and,
• Only includes research that validates assumptions and does not consider contrary findings and alternative interpretations found in the literature.

Cook, Kathleen E. and Elise Murowchick. “Do Literature Review Skills Transfer from One Course to Another?” Psychology Learning and Teaching 13 (March 2014): 3-11; Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Jesson, Jill. Doing Your Literature Review: Traditional and Systematic Techniques . London: SAGE, 2011; Literature Review Handout. Online Writing Center. Liberty University; Literature Reviews. The Writing Center. University of North Carolina; Onwuegbuzie, Anthony J. and Rebecca Frels. Seven Steps to a Comprehensive Literature Review: A Multimodal and Cultural Approach . Los Angeles, CA: SAGE, 2016; Ridley, Diana. The Literature Review: A Step-by-Step Guide for Students . 2nd ed. Los Angeles, CA: SAGE, 2012; Randolph, Justus J. “A Guide to Writing the Dissertation Literature Review." Practical Assessment, Research, and Evaluation. vol. 14, June 2009; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016; Taylor, Dena. The Literature Review: A Few Tips On Conducting It. University College Writing Centre. University of Toronto; Writing a Literature Review. Academic Skills Centre. University of Canberra.

Writing Tip

Break Out of Your Disciplinary Box!

Thinking interdisciplinarily about a research problem can be a rewarding exercise in applying new ideas, theories, or concepts to an old problem. For example, what might cultural anthropologists say about the continuing conflict in the Middle East? In what ways might geographers view the need for better distribution of social service agencies in large cities than how social workers might study the issue? You don’t want to substitute a thorough review of core research literature in your discipline for studies conducted in other fields of study. However, particularly in the social sciences, thinking about research problems from multiple vectors is a key strategy for finding new solutions to a problem or gaining a new perspective. Consult with a librarian about identifying research databases in other disciplines; almost every field of study has at least one comprehensive database devoted to indexing its research literature.

Frodeman, Robert. The Oxford Handbook of Interdisciplinarity . New York: Oxford University Press, 2010.

Another Writing Tip

Don't Just Review for Content!

While conducting a review of the literature, maximize the time you devote to writing this part of your paper by thinking broadly about what you should be looking for and evaluating. Review not just what scholars are saying, but how are they saying it. Some questions to ask:

• How are they organizing their ideas?
• What methods have they used to study the problem?
• What theories have been used to explain, predict, or understand their research problem?
• What sources have they cited to support their conclusions?
• How have they used non-textual elements [e.g., charts, graphs, figures, etc.] to illustrate key points?

When you begin to write your literature review section, you'll be glad you dug deeper into how the research was designed and constructed because it establishes a means for developing more substantial analysis and interpretation of the research problem.

Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1 998.

Yet Another Writing Tip

When Do I Know I Can Stop Looking and Move On?

Here are several strategies you can utilize to assess whether you've thoroughly reviewed the literature:

• Look for repeating patterns in the research findings . If the same thing is being said, just by different people, then this likely demonstrates that the research problem has hit a conceptual dead end. At this point consider: Does your study extend current research?  Does it forge a new path? Or, does is merely add more of the same thing being said?
• Look at sources the authors cite to in their work . If you begin to see the same researchers cited again and again, then this is often an indication that no new ideas have been generated to address the research problem.
• Search Google Scholar to identify who has subsequently cited leading scholars already identified in your literature review [see next sub-tab]. This is called citation tracking and there are a number of sources that can help you identify who has cited whom, particularly scholars from outside of your discipline. Here again, if the same authors are being cited again and again, this may indicate no new literature has been written on the topic.

Onwuegbuzie, Anthony J. and Rebecca Frels. Seven Steps to a Comprehensive Literature Review: A Multimodal and Cultural Approach . Los Angeles, CA: Sage, 2016; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016.

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Literature Review - what is a Literature Review, why it is important and how it is done

• Strategies to Find Sources

Evaluating Literature Reviews and Sources

Reading critically, tips to evaluate sources.

• Tips for Writing Literature Reviews
• Writing Literature Review: Useful Sites
• Citation Resources
• Other Academic Writings
• Useful Resources

A good literature review evaluates a wide variety of sources (academic articles, scholarly books, government/NGO reports). It also evaluates literature reviews that study similar topics. This page offers you a list of resources and tips on how to evaluate the sources that you may use to write your review.

• A Closer Look at Evaluating Literature Reviews Excerpt from the book chapter, “Evaluating Introductions and Literature Reviews” in Fred Pyrczak’s Evaluating Research in Academic Journals: A Practical Guide to Realistic Evaluation , (Chapter 4 and 5). This PDF discusses and offers great advice on how to evaluate "Introductions" and "Literature Reviews" by listing questions and tips. First part focus on Introductions and in page 10 in the PDF, 37 in the text, it focus on "literature reviews".
• Tips for Evaluating Sources (Print vs. Internet Sources) Excellent page that will guide you on what to ask to determine if your source is a reliable one. Check the other topics in the guide: Evaluating Bibliographic Citations and Evaluation During Reading on the left side menu.

To be able to write a good Literature Review, you need to be able to read critically. Below are some tips that will help you evaluate the sources for your paper.

Reading critically (summary from How to Read Academic Texts Critically)

• Who is the author? What is his/her standing in the field.
• What is the author’s purpose? To offer advice, make practical suggestions, solve a specific problem, to critique or clarify?
• Note the experts in the field: are there specific names/labs that are frequently cited?
• Pay attention to methodology: is it sound? what testing procedures, subjects, materials were used?
• Note conflicting theories, methodologies and results. Are there any assumptions being made by most/some researchers?
• Theories: have they evolved overtime?
• Evaluate and synthesize the findings and conclusions. How does this study contribute to your project?

Useful links:

• How to Read a Paper (University of Waterloo, Canada) This is an excellent paper that teach you how to read an academic paper, how to determine if it is something to set aside, or something to read deeply. Good advice to organize your literature for the Literature Review or just reading for classes.

Criteria to evaluate sources:

• Authority : Who is the author? what is his/her credentials--what university he/she is affliliated? Is his/her area of expertise?
• Usefulness : How this source related to your topic? How current or relevant it is to your topic?
• Reliability : Does the information comes from a reliable, trusted source such as an academic journal?

Useful site - Critically Analyzing Information Sources (Cornell University Library)

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• Next: Tips for Writing Literature Reviews >>
• Last Updated: Jul 3, 2024 10:56 AM
• URL: https://lit.libguides.com/Literature-Review

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How to Write a Literature Review: Six Steps to Get You from Start to Finish

Writing-a-literature-review-six-steps-to-get-you-from-start-to-finish.

Tanya Golash-Boza, Associate Professor of Sociology, University of California

February 03, 2022

Writing a literature review is often the most daunting part of writing an article, book, thesis, or dissertation. “The literature” seems (and often is) massive. I have found it helpful to be as systematic as possible when completing this gargantuan task.

Sonja Foss and William Walters* describe an efficient and effective way of writing a literature review. Their system provides an excellent guide for getting through the massive amounts of literature for any purpose: in a dissertation, an M.A. thesis, or preparing a research article for publication  in any field of study. Below is a  summary of the steps they outline as well as a step-by-step method for writing a literature review.

How to Write a Literature Review

Step One: Decide on your areas of research:

Before you begin to search for articles or books, decide beforehand what areas you are going to research. Make sure that you only get articles and books in those areas, even if you come across fascinating books in other areas. A literature review I am currently working on, for example, explores barriers to higher education for undocumented students.

Step Two: Search for the literature:

Conduct a comprehensive bibliographic search of books and articles in your area. Read the abstracts online and download and/or print those articles that pertain to your area of research. Find books in the library that are relevant and check them out. Set a specific time frame for how long you will search. It should not take more than two or three dedicated sessions.

Step Three: Find relevant excerpts in your books and articles:

Skim the contents of each book and article and look specifically for these five things:

1. Claims, conclusions, and findings about the constructs you are investigating

2. Definitions of terms

3. Calls for follow-up studies relevant to your project

4. Gaps you notice in the literature

5. Disagreement about the constructs you are investigating

When you find any of these five things, type the relevant excerpt directly into a Word document. Don’t summarize, as summarizing takes longer than simply typing the excerpt. Make sure to note the name of the author and the page number following each excerpt. Do this for each article and book that you have in your stack of literature. When you are done, print out your excerpts.

Step Four: Code the literature:

Get out a pair of scissors and cut each excerpt out. Now, sort the pieces of paper into similar topics. Figure out what the main themes are. Place each excerpt into a themed pile. Make sure each note goes into a pile. If there are excerpts that you can’t figure out where they belong, separate those and go over them again at the end to see if you need new categories. When you finish, place each stack of notes into an envelope labeled with the name of the theme.

Step Five: Create Your Conceptual Schema:

Type, in large font, the name of each of your coded themes. Print this out, and cut the titles into individual slips of paper. Take the slips of paper to a table or large workspace and figure out the best way to organize them. Are there ideas that go together or that are in dialogue with each other? Are there ideas that contradict each other? Move around the slips of paper until you come up with a way of organizing the codes that makes sense. Write the conceptual schema down before you forget or someone cleans up your slips of paper.

Step Six: Begin to Write Your Literature Review:

Choose any section of your conceptual schema to begin with. You can begin anywhere, because you already know the order. Find the envelope with the excerpts in them and lay them on the table in front of you. Figure out a mini-conceptual schema based on that theme by grouping together those excerpts that say the same thing. Use that mini-conceptual schema to write up your literature review based on the excerpts that you have in front of you. Don’t forget to include the citations as you write, so as not to lose track of who said what. Repeat this for each section of your literature review.

Once you complete these six steps, you will have a complete draft of your literature review. The great thing about this process is that it breaks down into manageable steps something that seems enormous: writing a literature review.

I think that Foss and Walter’s system for writing the literature review is ideal for a dissertation, because a Ph.D. candidate has already read widely in his or her field through graduate seminars and comprehensive exams.

It may be more challenging for M.A. students, unless you are already familiar with the literature. It is always hard to figure out how much you need to read for deep meaning, and how much you just need to know what others have said. That balance will depend on how much you already know.

For people writing literature reviews for articles or books, this system also could work, especially when you are writing in a field with which you are already familiar. The mere fact of having a system can make the literature review seem much less daunting, so I recommend this system for anyone who feels overwhelmed by the prospect of writing a literature review.

*Destination Dissertation: A Traveler's Guide to a Done Dissertation

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Organizing and Creating Information

• Citation and Attribution

What Is a Literature Review?

Review the literature, write the literature review, further reading, learning objectives, attribution.

This guide is designed to:

• Identify the sections and purpose of a literature review in academic writing
• Review practical strategies and organizational methods for preparing a literature review

A literature review is a summary and synthesis of scholarly research on a specific topic. It should answer questions such as:

• What research has been done on the topic?
• Who are the key researchers and experts in the field?
• What are the common theories and methodologies?
• Are there challenges, controversies, and contradictions?
• Are there gaps in the research that your approach addresses?

The process of reviewing existing research allows you to fine-tune your research question and contextualize your own work. Preparing a literature review is a cyclical process. You may find that the research question you begin with evolves as you learn more about the topic.

Once you have defined your research question , focus on learning what other scholars have written on the topic.

In order to  do a thorough search of the literature  on the topic, define the basic criteria:

• Databases and journals: Look at the  subject guide  related to your topic for recommended databases. Review the  tutorial on finding articles  for tips. 
• Books: Search BruKnow, the Library's catalog. Steps to searching ebooks are covered in the  Finding Ebooks tutorial .
• What time period should it cover? Is currency important?
• Do I know of primary and secondary sources that I can use as a way to find other information?
• What should I be aware of when looking at popular, trade, and scholarly resources ? 

One strategy is to review bibliographies for sources that relate to your interest. For more on this technique, look at the tutorial on finding articles when you have a citation .

Tip: Use a Synthesis Matrix

As you read sources, themes will emerge that will help you to organize the review. You can use a simple Synthesis Matrix to track your notes as you read. From this work, a concept map emerges that provides an overview of the literature and ways in which it connects. Working with Zotero to capture the citations, you build the structure for writing your literature review.

How do I know when I am done?

A key indicator for knowing when you are done is running into the same articles and materials. With no new information being uncovered, you are likely exhausting your current search and should modify search terms or search different catalogs or databases. It is also possible that you have reached a point when you can start writing the literature review.

Tip: Manage Your Citations

These citation management tools also create citations, footnotes, and bibliographies with just a few clicks:

Zotero Tutorial

Endnote Tutorial

Your literature review should be focused on the topic defined in your research question. It should be written in a logical, structured way and maintain an objective perspective and use a formal voice.

Review the Summary Table you created for themes and connecting ideas. Use the following guidelines to prepare an outline of the main points you want to make. 

• Synthesize previous research on the topic.
• Aim to include both summary and synthesis.
• Include literature that supports your research question as well as that which offers a different perspective.
• Avoid relying on one author or publication too heavily.
• Select an organizational structure, such as chronological, methodological, and thematic.

The three elements of a literature review are introduction, body, and conclusion.

Introduction

• Define the topic of the literature review, including any terminology.
• Introduce the central theme and organization of the literature review.
• Summarize the state of research on the topic.
• Frame the literature review with your research question.
• Focus on ways to have the body of literature tell its own story. Do not add your own interpretations at this point.
• Look for patterns and find ways to tie the pieces together.
• Summarize instead of quote.
• Weave the points together rather than list summaries of each source.
• Include the most important sources, not everything you have read.
• Summarize the review of the literature.
• Identify areas of further research on the topic.
• Connect the review with your research.
• DeCarlo, M. (2018). 4.1 What is a literature review? In Scientific Inquiry in Social Work. Open Social Work Education. https://scientificinquiryinsocialwork.pressbooks.com/chapter/4-1-what-is-a-literature-review/
• Literature Reviews (n.d.) https://writingcenter.unc.edu/tips-and-tools/literature-reviews/ Accessed Nov. 10, 2021

This guide was designed to: 

• Identify the sections and purpose of a literature review in academic writing 
• Review practical strategies and organizational methods for preparing a literature review​

Content on this page adapted from: 

Frederiksen, L. and Phelps, S. (2017).   Literature Reviews for Education and Nursing Graduate Students.  Licensed CC BY 4.0

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• Last Updated: Jul 17, 2024 3:55 PM
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Literature Reviews

What this handout is about.

This handout will explain what literature reviews are and offer insights into the form and construction of literature reviews in the humanities, social sciences, and sciences.

Introduction

OK. You’ve got to write a literature review. You dust off a novel and a book of poetry, settle down in your chair, and get ready to issue a “thumbs up” or “thumbs down” as you leaf through the pages. “Literature review” done. Right?

Wrong! The “literature” of a literature review refers to any collection of materials on a topic, not necessarily the great literary texts of the world. “Literature” could be anything from a set of government pamphlets on British colonial methods in Africa to scholarly articles on the treatment of a torn ACL. And a review does not necessarily mean that your reader wants you to give your personal opinion on whether or not you liked these sources.

What is a literature review, then?

A literature review discusses published information in a particular subject area, and sometimes information in a particular subject area within a certain time period.

A literature review can be just a simple summary of the sources, but it usually has an organizational pattern and combines both summary and synthesis. A summary is a recap of the important information of the source, but a synthesis is a re-organization, or a reshuffling, of that information. It might give a new interpretation of old material or combine new with old interpretations. Or it might trace the intellectual progression of the field, including major debates. And depending on the situation, the literature review may evaluate the sources and advise the reader on the most pertinent or relevant.

But how is a literature review different from an academic research paper?

The main focus of an academic research paper is to develop a new argument, and a research paper is likely to contain a literature review as one of its parts. In a research paper, you use the literature as a foundation and as support for a new insight that you contribute. The focus of a literature review, however, is to summarize and synthesize the arguments and ideas of others without adding new contributions.

Why do we write literature reviews?

Literature reviews provide you with a handy guide to a particular topic. If you have limited time to conduct research, literature reviews can give you an overview or act as a stepping stone. For professionals, they are useful reports that keep them up to date with what is current in the field. For scholars, the depth and breadth of the literature review emphasizes the credibility of the writer in his or her field. Literature reviews also provide a solid background for a research paper’s investigation. Comprehensive knowledge of the literature of the field is essential to most research papers.

Who writes these things, anyway?

Literature reviews are written occasionally in the humanities, but mostly in the sciences and social sciences; in experiment and lab reports, they constitute a section of the paper. Sometimes a literature review is written as a paper in itself.

Let’s get to it! What should I do before writing the literature review?

If your assignment is not very specific, seek clarification from your instructor:

• Roughly how many sources should you include?
• What types of sources (books, journal articles, websites)?
• Should you summarize, synthesize, or critique your sources by discussing a common theme or issue?
• Should you evaluate your sources?
• Should you provide subheadings and other background information, such as definitions and/or a history?

Find models

Look for other literature reviews in your area of interest or in the discipline and read them to get a sense of the types of themes you might want to look for in your own research or ways to organize your final review. You can simply put the word “review” in your search engine along with your other topic terms to find articles of this type on the Internet or in an electronic database. The bibliography or reference section of sources you’ve already read are also excellent entry points into your own research.

Narrow your topic

There are hundreds or even thousands of articles and books on most areas of study. The narrower your topic, the easier it will be to limit the number of sources you need to read in order to get a good survey of the material. Your instructor will probably not expect you to read everything that’s out there on the topic, but you’ll make your job easier if you first limit your scope.

Keep in mind that UNC Libraries have research guides and to databases relevant to many fields of study. You can reach out to the subject librarian for a consultation: https://library.unc.edu/support/consultations/ .

And don’t forget to tap into your professor’s (or other professors’) knowledge in the field. Ask your professor questions such as: “If you had to read only one book from the 90’s on topic X, what would it be?” Questions such as this help you to find and determine quickly the most seminal pieces in the field.

Consider whether your sources are current

Some disciplines require that you use information that is as current as possible. In the sciences, for instance, treatments for medical problems are constantly changing according to the latest studies. Information even two years old could be obsolete. However, if you are writing a review in the humanities, history, or social sciences, a survey of the history of the literature may be what is needed, because what is important is how perspectives have changed through the years or within a certain time period. Try sorting through some other current bibliographies or literature reviews in the field to get a sense of what your discipline expects. You can also use this method to consider what is currently of interest to scholars in this field and what is not.

Strategies for writing the literature review

Find a focus.

A literature review, like a term paper, is usually organized around ideas, not the sources themselves as an annotated bibliography would be organized. This means that you will not just simply list your sources and go into detail about each one of them, one at a time. No. As you read widely but selectively in your topic area, consider instead what themes or issues connect your sources together. Do they present one or different solutions? Is there an aspect of the field that is missing? How well do they present the material and do they portray it according to an appropriate theory? Do they reveal a trend in the field? A raging debate? Pick one of these themes to focus the organization of your review.

Convey it to your reader

A literature review may not have a traditional thesis statement (one that makes an argument), but you do need to tell readers what to expect. Try writing a simple statement that lets the reader know what is your main organizing principle. Here are a couple of examples:

The current trend in treatment for congestive heart failure combines surgery and medicine. More and more cultural studies scholars are accepting popular media as a subject worthy of academic consideration.

Consider organization

You’ve got a focus, and you’ve stated it clearly and directly. Now what is the most effective way of presenting the information? What are the most important topics, subtopics, etc., that your review needs to include? And in what order should you present them? Develop an organization for your review at both a global and local level:

First, cover the basic categories

Just like most academic papers, literature reviews also must contain at least three basic elements: an introduction or background information section; the body of the review containing the discussion of sources; and, finally, a conclusion and/or recommendations section to end the paper. The following provides a brief description of the content of each:

• Introduction: Gives a quick idea of the topic of the literature review, such as the central theme or organizational pattern.
• Body: Contains your discussion of sources and is organized either chronologically, thematically, or methodologically (see below for more information on each).
• Conclusions/Recommendations: Discuss what you have drawn from reviewing literature so far. Where might the discussion proceed?

Organizing the body

Once you have the basic categories in place, then you must consider how you will present the sources themselves within the body of your paper. Create an organizational method to focus this section even further.

To help you come up with an overall organizational framework for your review, consider the following scenario:

You’ve decided to focus your literature review on materials dealing with sperm whales. This is because you’ve just finished reading Moby Dick, and you wonder if that whale’s portrayal is really real. You start with some articles about the physiology of sperm whales in biology journals written in the 1980’s. But these articles refer to some British biological studies performed on whales in the early 18th century. So you check those out. Then you look up a book written in 1968 with information on how sperm whales have been portrayed in other forms of art, such as in Alaskan poetry, in French painting, or on whale bone, as the whale hunters in the late 19th century used to do. This makes you wonder about American whaling methods during the time portrayed in Moby Dick, so you find some academic articles published in the last five years on how accurately Herman Melville portrayed the whaling scene in his novel.

Now consider some typical ways of organizing the sources into a review:

• Chronological: If your review follows the chronological method, you could write about the materials above according to when they were published. For instance, first you would talk about the British biological studies of the 18th century, then about Moby Dick, published in 1851, then the book on sperm whales in other art (1968), and finally the biology articles (1980s) and the recent articles on American whaling of the 19th century. But there is relatively no continuity among subjects here. And notice that even though the sources on sperm whales in other art and on American whaling are written recently, they are about other subjects/objects that were created much earlier. Thus, the review loses its chronological focus.
• By publication: Order your sources by publication chronology, then, only if the order demonstrates a more important trend. For instance, you could order a review of literature on biological studies of sperm whales if the progression revealed a change in dissection practices of the researchers who wrote and/or conducted the studies.
• By trend: A better way to organize the above sources chronologically is to examine the sources under another trend, such as the history of whaling. Then your review would have subsections according to eras within this period. For instance, the review might examine whaling from pre-1600-1699, 1700-1799, and 1800-1899. Under this method, you would combine the recent studies on American whaling in the 19th century with Moby Dick itself in the 1800-1899 category, even though the authors wrote a century apart.
• Thematic: Thematic reviews of literature are organized around a topic or issue, rather than the progression of time. However, progression of time may still be an important factor in a thematic review. For instance, the sperm whale review could focus on the development of the harpoon for whale hunting. While the study focuses on one topic, harpoon technology, it will still be organized chronologically. The only difference here between a “chronological” and a “thematic” approach is what is emphasized the most: the development of the harpoon or the harpoon technology.But more authentic thematic reviews tend to break away from chronological order. For instance, a thematic review of material on sperm whales might examine how they are portrayed as “evil” in cultural documents. The subsections might include how they are personified, how their proportions are exaggerated, and their behaviors misunderstood. A review organized in this manner would shift between time periods within each section according to the point made.
• Methodological: A methodological approach differs from the two above in that the focusing factor usually does not have to do with the content of the material. Instead, it focuses on the “methods” of the researcher or writer. For the sperm whale project, one methodological approach would be to look at cultural differences between the portrayal of whales in American, British, and French art work. Or the review might focus on the economic impact of whaling on a community. A methodological scope will influence either the types of documents in the review or the way in which these documents are discussed. Once you’ve decided on the organizational method for the body of the review, the sections you need to include in the paper should be easy to figure out. They should arise out of your organizational strategy. In other words, a chronological review would have subsections for each vital time period. A thematic review would have subtopics based upon factors that relate to the theme or issue.

Sometimes, though, you might need to add additional sections that are necessary for your study, but do not fit in the organizational strategy of the body. What other sections you include in the body is up to you. Put in only what is necessary. Here are a few other sections you might want to consider:

• Current Situation: Information necessary to understand the topic or focus of the literature review.
• History: The chronological progression of the field, the literature, or an idea that is necessary to understand the literature review, if the body of the literature review is not already a chronology.
• Methods and/or Standards: The criteria you used to select the sources in your literature review or the way in which you present your information. For instance, you might explain that your review includes only peer-reviewed articles and journals.

Questions for Further Research: What questions about the field has the review sparked? How will you further your research as a result of the review?

Begin composing

Once you’ve settled on a general pattern of organization, you’re ready to write each section. There are a few guidelines you should follow during the writing stage as well. Here is a sample paragraph from a literature review about sexism and language to illuminate the following discussion:

However, other studies have shown that even gender-neutral antecedents are more likely to produce masculine images than feminine ones (Gastil, 1990). Hamilton (1988) asked students to complete sentences that required them to fill in pronouns that agreed with gender-neutral antecedents such as “writer,” “pedestrian,” and “persons.” The students were asked to describe any image they had when writing the sentence. Hamilton found that people imagined 3.3 men to each woman in the masculine “generic” condition and 1.5 men per woman in the unbiased condition. Thus, while ambient sexism accounted for some of the masculine bias, sexist language amplified the effect. (Source: Erika Falk and Jordan Mills, “Why Sexist Language Affects Persuasion: The Role of Homophily, Intended Audience, and Offense,” Women and Language19:2).

Use evidence

In the example above, the writers refer to several other sources when making their point. A literature review in this sense is just like any other academic research paper. Your interpretation of the available sources must be backed up with evidence to show that what you are saying is valid.

Be selective

Select only the most important points in each source to highlight in the review. The type of information you choose to mention should relate directly to the review’s focus, whether it is thematic, methodological, or chronological.

Use quotes sparingly

Falk and Mills do not use any direct quotes. That is because the survey nature of the literature review does not allow for in-depth discussion or detailed quotes from the text. Some short quotes here and there are okay, though, if you want to emphasize a point, or if what the author said just cannot be rewritten in your own words. Notice that Falk and Mills do quote certain terms that were coined by the author, not common knowledge, or taken directly from the study. But if you find yourself wanting to put in more quotes, check with your instructor.

Summarize and synthesize

Remember to summarize and synthesize your sources within each paragraph as well as throughout the review. The authors here recapitulate important features of Hamilton’s study, but then synthesize it by rephrasing the study’s significance and relating it to their own work.

Keep your own voice

While the literature review presents others’ ideas, your voice (the writer’s) should remain front and center. Notice that Falk and Mills weave references to other sources into their own text, but they still maintain their own voice by starting and ending the paragraph with their own ideas and their own words. The sources support what Falk and Mills are saying.

Use caution when paraphrasing

When paraphrasing a source that is not your own, be sure to represent the author’s information or opinions accurately and in your own words. In the preceding example, Falk and Mills either directly refer in the text to the author of their source, such as Hamilton, or they provide ample notation in the text when the ideas they are mentioning are not their own, for example, Gastil’s. For more information, please see our handout on plagiarism .

Revise, revise, revise

Draft in hand? Now you’re ready to revise. Spending a lot of time revising is a wise idea, because your main objective is to present the material, not the argument. So check over your review again to make sure it follows the assignment and/or your outline. Then, just as you would for most other academic forms of writing, rewrite or rework the language of your review so that you’ve presented your information in the most concise manner possible. Be sure to use terminology familiar to your audience; get rid of unnecessary jargon or slang. Finally, double check that you’ve documented your sources and formatted the review appropriately for your discipline. For tips on the revising and editing process, see our handout on revising drafts .

Works consulted

We consulted these works while writing this handout. This is not a comprehensive list of resources on the handout’s topic, and we encourage you to do your own research to find additional publications. Please do not use this list as a model for the format of your own reference list, as it may not match the citation style you are using. For guidance on formatting citations, please see the UNC Libraries citation tutorial . We revise these tips periodically and welcome feedback.

Anson, Chris M., and Robert A. Schwegler. 2010. The Longman Handbook for Writers and Readers , 6th ed. New York: Longman.

Jones, Robert, Patrick Bizzaro, and Cynthia Selfe. 1997. The Harcourt Brace Guide to Writing in the Disciplines . New York: Harcourt Brace.

Lamb, Sandra E. 1998. How to Write It: A Complete Guide to Everything You’ll Ever Write . Berkeley: Ten Speed Press.

Rosen, Leonard J., and Laurence Behrens. 2003. The Allyn & Bacon Handbook , 5th ed. New York: Longman.

Troyka, Lynn Quittman, and Doug Hesse. 2016. Simon and Schuster Handbook for Writers , 11th ed. London: Pearson.

You may reproduce it for non-commercial use if you use the entire handout and attribute the source: The Writing Center, University of North Carolina at Chapel Hill

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Literature Review How To

• Things To Consider
• Synthesizing Sources
• Video Tutorials
• Books On Literature Reviews

Consider This

• What is the specific thesis, problem, or research question that my literature review aims to define?
• What type of literature review am I conducting? Am I looking at issues of theory, methodology, policy, quantitative research (e.g. interviews, observations), qualitative research (e.g., studies, surveys, statistics)?
• What is the scope of my literature review? What types of publications am I using (e.g., journals, books, government documents)?
• What discipline am I working in (e.g. Public Health, Nursing, Kinesiology etc.)?
• Has my search for sources been wide enough to ensure that I have found all the relevant material?
• Has it been narrow enough to exclude irrelevant material?
• Is the number of sources I've used appropriate for the length of my paper (i.e. if your literature review is part of a larger paper or assignment)?
• Have I critically analyzed the resources I found?
• How will I avoid just listing and summarizing resources? Do I assess them, discussing strengths and weaknesses?
• Have I cited and discussed studies contrary to my perspective?
• Will the reader find my literature review relevant, appropriate, and or useful?

FIRST, ASK YOUR PROFESSOR!

The format of a literature review may vary from discipline to discipline and from assignment to assignment. However, a literature review must do these things:

• Be organized around and related directly to the thesis or research question you are developing
• Synthesize results into a summary of what is and is not known
• Identify problematic areas or areas of controversy in the literature
• Formulate questions or issues that need further research

Remember! A literature review is not a list describing or summarizing one piece of literature after another.

Try and avoid starting every paragraph with the name of a researcher or the title of the work. Rather, try organizing the literature review into sections that present themes or identify trends, including relevant theories. You are not trying to list all the material published on a topic, but to synthesize and evaluate it according to the guiding concept of your thesis or research question.

Consider These For Each Source

• Has the author formulated a problem/issue?
• Is the problem/issue clearly defined and is its significance (scope, severity, and relevance) clearly established?
• Could the problem/issue have been approached more effectively and or from another perspective?
• What is the author's research orientation (e.g., interpretive, critical science, combination)?
• What is the author's theoretical framework (e.g., psychological, developmental, feminist)?
• What is the relationship between #4 and #5?
• Has the author evaluated the literature relevant to the topic (i.e. does the author include a literature review and or provide sources that take positions she/he does not agree with)?
• How accurate and valid are the measurements, statistics or data the author has provided?
• Is the analysis of the measurements, statistics or data accurate and relevant to the research question?
• Are the conclusions validly based upon the data and analysis?
• · How does the author structure the argument? Can you retrace the steps he/she takes and analyze the flow of the argument to see if it progresses logically?
• In what ways does this piece contribute to our understanding of the topic, and in what ways is it useful? What are the strengths and limitations?
• How does this book or article relate to my thesis or research question?
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Literature Reviews

• What is a Literature Review?
• Steps for Creating a Literature Review
• Providing Evidence / Critical Analysis
• Challenges when writing a Literature Review
• Systematic Literature Reviews

Developing a Literature Review

1. Purpose and Scope

To help you develop a literature review, gather information on existing research, sub-topics, relevant research, and overlaps. Note initial thoughts on the topic - a mind map or list might be helpful - and avoid unfocused reading, collecting irrelevant content.  A literature review serves to place your research within the context of existing knowledge. It demonstrates your understanding of the field and identifies gaps that your research aims to fill. This helps in justifying the relevance and necessity of your study.

To avoid over-reading, set a target word count for each section and limit reading time. Plan backwards from the deadline and move on to other parts of the investigation. Read major texts and explore up-to-date research. Check reference lists and citation indexes for common standard texts. Be guided by research questions and refocus on your topic when needed. Stop reading if you find similar viewpoints or if you're going off topic.

You can use a "Synthesis Matrix" to keep track of your reading notes. This concept map helps you to provide a summary of the literature and its connections is produced as a result of this study. Utilizing referencing software like RefWorks to obtain citations, you can construct the framework for composing your literature evaluation.

2. Source Selection

Focus on searching for academically authoritative texts such as academic books, journals, research reports, and government publications. These sources are critical for ensuring the credibility and reliability of your review. 

• Academic Books: Provide comprehensive coverage of a topic.
• Journal Articles: Offer the most up-to-date research and are essential for a literature review.
• Research Reports: Detailed accounts of specific research projects.
• Government Publications: Official documents that provide reliable data and insights.

3. Thematic Analysis

Instead of merely summarizing sources, identify and discuss key themes that emerge from the literature. This involves interpreting and evaluating how different authors have tackled similar issues and how their findings relate to your research.

4. Critical Evaluation

Adopt a critical attitude towards the sources you review. Scrutinize, question, and dissect the material to ensure that your review is not just descriptive but analytical. This helps in highlighting the significance of various sources and their relevance to your research.

Each work's critical assessment should take into account:

Provenance:  What qualifications does the author have? Are the author's claims backed up by proof, such as first-hand accounts from history, case studies, stories, statistics, and current scientific discoveries? Methodology:  Were the strategies employed to locate, collect, and evaluate the data suitable for tackling the study question? Was the sample size suitable? Were the findings properly reported and interpreted? Objectivity : Is the author's viewpoint impartial or biased? Does the author's thesis get supported by evidence that refutes it, or does it ignore certain important facts? Persuasiveness:  Which of the author's arguments is the strongest or weakest in terms of persuasiveness? Value:  Are the author's claims and deductions believable? Does the study ultimately advance our understanding of the issue in any meaningful way?

5. Categorization

Organize your literature review by grouping sources into categories based on themes, relevance to research questions, theoretical paradigms, or chronology. This helps in presenting your findings in a structured manner.

6. Source Validity

Ensure that the sources you include are valid and reliable. Classic texts may retain their authority over time, but for fields that evolve rapidly, prioritize the most recent research. Always check the credibility of the authors and the impact of their work in the field.

7. Synthesis and Findings

Synthesize the information from various sources to draw conclusions about the current state of knowledge. Identify trends, controversies, and gaps in the literature. Relate your findings to your research questions and suggest future directions for research.

Practical Tips

• Use a variety of sources, including online databases, university libraries, and reference lists from relevant articles. This ensures a comprehensive coverage of the literature.
• Avoid listing sources without analysis. Use tables, bulk citations, and footnotes to manage references efficiently and make your review more readable.
• Writing a literature review is an ongoing process. Start writing early and revise as you read more. This iterative process helps in refining your arguments and identifying additional sources as needed.  

Brown University Library (2024) Organizing and Creating Information. Available at: https://libguides.brown.edu/organize/litreview (Accessed: 30 July 2024).

Pacheco-Vega, R. (2016) Synthesizing different bodies of work in your literature review: The Conceptual Synthesis Excel Dump (CSED) technique . Available at: http://www.raulpacheco.org/2016/06/synthesizing-different-bodies-of-work-in-your-literature-review-the-conceptual-synthesis-excel-dump-technique/ (Accessed: 30 July 2024).

Study Advice at the University of Reading (2024) Literature reviews . Available at: https://libguides.reading.ac.uk/literaturereview/developing (Accessed: 31 July 2024).

Further Reading

Frameworks for creating answerable (re)search questions  How to Guide

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Scholarly Articles: How can I tell?

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Literature Review

• Author and affiliation
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The literature review section of an article is a summary or analysis of all the research the author read before doing his/her own research. This section may be part of the introduction or in a section called Background. It provides the background on who has done related research, what that research has or has not uncovered and how the current research contributes to the conversation on the topic. When you read the lit review ask:

• Does the review of the literature logically lead up to the research questions?
• Do the authors review articles relevant to their research study?
• Do the authors show where there are gaps in the literature?

The lit review is also a good place to find other sources you may want to read on this topic to help you get the bigger picture.

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Use of Phosphorus-Solubilizing Microorganisms as a Biotechnological Alternative: A Review

Efrén venancio ramos cabrera.

1 Escuela de Ciencias Agrícolas, Pecuarias y del Medio Ambiente—ECAPMA, Universidad Nacional Abierta y a Distancia—UNAD, Calle 5 # 46N-67, Popayán 190001, Cauca, Colombia; [email protected]

Zuly Yuliana Delgado Espinosa

2 Facultad de Ingeniería, Corporación Universitaria Comfacauca—Unicomfacauca, Cl. 4 N. 8-30, Popayán 190001, Cauca, Colombia; oc.ude.acuacafmocinu@odagledz

Andrés Felipe Solis Pino

3 Facultad de Ingeniería Electrónica y Telecomunicaciones, Universidad del Cauca, Popayán 190003, Cauca, Colombia

Associated Data

Using the following link, you can access the material to perform the bibliometric analysis and the literature review, as well as the materials used in this research ( https://acortar.link/UP1LXX , accessed on 22 June 2024).

Microorganisms with the ability to dissolve phosphorus have the potential to release this essential nutrient into the soil through natural solubilization processes, which allows for boosting plant growth and development. While literature reviews acknowledge their potential, unexplored territories concerning accessibility, application, and effective integration into sustainable agriculture necessitate further research. This manuscript employed distinct methodologies to execute a bibliometric analysis and a literature review. The combined application of both methodologies enables a holistic understanding of the domain landscape and its innovative facets. For the bibliometric analysis, the propositions of Donthu and Jia were utilized, supplemented by tools, such as Bibliometrix. The literature review adhered to a systematic methodology predicated on Petersen’s guidelines to represent the domain accurately, pinpointing trends and gaps that could steer future, more detailed research. This investigation uncovers an escalating interest in studying these microorganisms since the 2000s, emphasizing their significance in sustainable agriculture and the context of phosphorus scarcity. It was also discerned that India and China, nations with notable agricultural sectors and a high demand for phosphorus fertilizers, spearheaded research output on this subject. This signifies their substantial contribution to the progression of this scientific field. Furthermore, according to the research consulted, phosphorus-solubilizing microorganisms play a pivotal role in the symbiotic interaction of soil with plant roots and represent an efficacious strategy to counteract the low availability of phosphorus in the soil and sustainably enhance agricultural systems. Finally, this review contributes to the relevant domain by examining existing empirical evidence with special emphasis on sustainable agriculture, improved understanding of phosphorus solubilization mechanisms, and recognition of various microbial entities.

1. Introduction

Phosphorus is an essential nutrient for plant growth and development, playing a crucial role in plant physiology and reproduction. It is present in the fundamental molecules of life, such as nucleic acids, phospholipids, Adenosine Triphosphate (ATP), and other indispensable biological substances [ 1 , 2 ]. Plants’ demand for phosphorus is the second highest after nitrogen, limiting agricultural productivity and production [ 3 ]. Although soil may contain a significant amount of phosphorus, its uptake is restricted by soil chemical conditions, such as pH, and high concentrations of elements, such as aluminum and iron [ 4 ]. Phosphorus is highly reactive and soluble in a narrow pH range (neutral to slightly acidic) [ 5 ]. In acid soils, phosphorus complexes with aluminum and iron, whereas in alkaline soils, it complexes with calcium and magnesium to form insoluble phosphate compounds that plants cannot assimilate due to their cation and ammonium exchange capacity [ 6 ]. This limitation represents one of the main challenges for agricultural production, as it directly impacts crop yield [ 3 ].

Fertilizers are fundamental to increasing agricultural productivity by providing plants with the essential nutrients necessary for their growth and development [ 7 ]. These can be of natural origin, chemically synthesized, organic, or inorganic and contain nitrogen, phosphorus, potassium, calcium, sulfur, and magnesium [ 8 ]. Fertilizers help improve soil fertility, increase crop yields, improve product quality, and prevent plant nutrient deficiencies [ 9 ]. Globally, it is estimated that about 30 million tons of phosphate fertilizers are used to address phosphorus deficiency and improve crop productivity [ 10 ]. However, up to 80% of the phosphorus in these fertilizers is lost due to immobilization in soil colloids, preventing its conversion into plant-assimilable organic forms [ 11 , 12 ].

In recent years, extensive research has been conducted to address this problem by studying the excessive use of chemically synthesized phosphorus fertilizers, which often contain heavy metals, such as cadmium and lead [ 13 ]. In addition, these fertilizers contain strong acids, such as nitric acid, sulfuric acid, and hydrochloric acid, which can accumulate in the soil and reduce the beneficial microbial population, increasing soil acidity and ultimately affecting plant growth [ 14 ]. Therefore, biotechnological alternatives are currently being investigated, such as the search for microorganisms capable of solubilizing phosphate present in the soil into insoluble forms, which can improve the sustainability of agriculture by avoiding the need to expand the use of phosphorus chemical fertilizers that pollute the environment and cause effects, such as eutrophication or risks to human health [ 15 ].

Various studies and reviews have focused on understanding the role of phosphorus-solubilizing microorganisms and ways to improve their effectiveness in specific contexts. For instance, Kour in [ 16 ] highlights the significance of phosphorus as a crucial macronutrient for plant development and its limited mobility in plants and soil, affecting plant growth. The use of phosphorus solubilizers in combination with other plant-growth-promoting microbes is suggested to enhance phosphorus uptake in crops. Another perspective in [ 17 ] discusses the adverse effects of conventional phosphorus fertilizers and proposes inoculating seeds, crops, and soils with phosphate-solubilizing bacteria (PSB) as a promising strategy to improve food production without harming the environment. This review explores various soil microorganisms capable of solubilizing phosphorus and their potential as biofertilizers. Additionally, Sattar et al. [ 18 ] emphasize the importance of potassium as an essential nutrient for plants and its role in various physiological and metabolic processes.

Along this line, intensive cultivation practices have led to the depletion of soil fertility, particularly phosphorus, affecting the balance and diversity of soil microorganisms. Many of these microorganisms are phosphorus solubilizers that utilize organic acid production, pH lowering, acidolysis, chelation, and exchange reactions to convert unavailable phosphorus into accessible forms. Consequently, several phosphorus-solubilizing microorganisms are being marketed as biofertilizers to reduce reliance on chemical fertilizers and better use phosphorus reserves in soils.

Considering those described above, this investigation distinguishes itself from analogous inquiries by amalgamating two disparate yet complementary methodologies: bibliometric analysis and a literature review. This fusion provides a comprehensive viewpoint on the current state of phosphorus-solubilizing microorganisms as a biotechnological strategy for augmenting phosphorus accessibility. The extensive temporal range of the scrutinized studies, focused on revealing underlying mechanisms and delineating future research pathways, sets this manuscript apart from its contemporaries. Consequently, this endeavor furnishes substantial and meaningful advancements toward the progression and understanding of this scientific discipline.

The novelty of this study lies in its comprehensive approach, combining bibliometric analysis with an in-depth literature review to provide a holistic understanding of phosphorus-solubilizing microorganisms as a biotechnological strategy. This dual methodology allows for a quantitative assessment of research trends and a qualitative exploration of the mechanisms and applications of these microorganisms. The motivation for this work stems from the urgent need to address global phosphorus deficiency in agriculture while minimizing environmental impact. This study aims to accelerate the development and implementation of sustainable phosphorus management strategies in agriculture by synthesizing current knowledge and identifying research gaps.

The literature review revealed that similar systematic reviews or mappings do not supplement their reviews with bibliometric scrutiny. This oversight engenders a noticeable void in our comprehension, originating from the need for more application of statistical and mathematical methodologies to examine and construct metrics related to the dynamic and evolutionary patterns of scientific and technological data within the domain of solubilizing bacteria. The current investigation aims to bridge this knowledge gap by undertaking such an analysis.

2. Review Execution

This section of the paper outlines the systematic planning and execution of a bibliometric analysis and literature review focused on utilizing solubilizing microorganisms to enhance plant phosphorus availability. The bibliometric analysis primarily examines the authors, sources, and relevant countries associated with this subject [ 19 ]. Conversely, the literature review focuses on microorganisms and their role in improving plant phosphorus availability. By employing these techniques, a precise and methodical representation of the current state of research in this field is achieved. The analysis also helps to identify trends, areas of interest, and gaps that warrant further investigation [ 20 ].

The approach proposed by Donthu in [ 20 ] was adopted as the primary reference for the bibliometric analysis. Donthu’s proposal provides essential guidelines and instructions for conducting such studies and follows the Science Mapping Workflow methodology, which facilitates data acquisition, analysis, and visualization [ 21 ]. Additionally, the study incorporates the insights from Jia’s paper in [ 22 ], serving as a foundation for analyzing solubilizing microorganisms.

The literature review plan was executed systematically and reproducibly, ensuring organized information collection. This review adhered to the methodological process of Petersen’s guidelines in [ 23 ]. Several essential tools were employed to carry out this research. R-studio, which integrates the Bibliometrix library [ 24 ], was used to facilitate the execution of the bibliometric study. VOSviewer [ 25 ] was used to explore co-occurrence networks, employing text mining techniques to examine various inter-domain relationships. Finally, Google Sheets [ 25 ] and R-studio were used to generate the research graphs.

2.1. Information Collection

A search strategy was applied to several bibliographic databases to collect relevant information for the bibliometric analysis and literature review. The search strategy, outlined in Table 1 , was constructed based on the PICOC criteria [ 26 ]. These criteria ensure that the search string logically and methodically covers all topics in the research scope. For this study, data were extracted from databases, such as Scopus, Web of Science, Dimensions AI, and ScienceDirect, and a manual search was performed using Google Scholar. These sources are widely accepted for searching the scientific literature in engineering and other research areas [ 27 ].

Search strategy applied to different scientific databases.

The search string used in this study was divided into several sections to narrow the search within the domain of solubilizing bacteria and their role in enhancing phosphorus assimilation in plants. The initial part of the search focused on phosphorus as the research’s central element and critical point. The following section examined fertilizers and their application to improve plant conditions. The third part of the search chain specified the specific area of interest, namely agriculture and cultivation. Finally, the fourth stage investigated plants’ desired action of phosphorus assimilation.

2.2. Definition of the Research Questions

The research questions posed for the systematic literature review seek to understand in what context phosphorus-solubilizing microorganisms are being used to improve the ability of plants to assimilate this mineral. This makes it possible to find the existing deficiencies in the research area and how to improve this process. This is intended to identify gaps or deficiencies in the research area and propose methods to improve this process. Table 2 outlines the individual research questions and their underlying motivations.

Research questions used in the literature review.

2.3. Methodology and Selection of Primary Studies

A methodological and systematic approach was adopted to ensure a rigorous and structured literature review, following some of the guidelines proposed by Petersen in [ 23 ], as they provide a clear framework for defining the scope of the review and, consequently, the primary studies. Figure 1 illustrates the information collection process, allowing us to address the studies considered effectively.

The methodology used in the literature review.

2.4. Execution of the Search

Research studies were selected with precision and systematic rigor through a tripartite methodology. A bespoke search query was formulated and applied to the titles, abstracts, and keywords across many databases in the initial phase. This preliminary search yielded 2322 scholarly articles, from which articles and conference papers were retained while content deemed irrelevant was excluded. A manual selection was also performed using Google Scholar to refine the selection, resulting in a subset of pertinent articles. This subset was subsequently reduced to 186 “candidate primary studies” based on examining their titles and abstracts. In the second phase, a similar methodology was employed, but with the acquisition and comprehensive reading of the full texts of the articles. This reading approach facilitated the identification of 155 primary articles that satisfied the inclusion criteria. The final phase involved systematically extracting relevant information from these 155 primary articles to address the research questions. An online tool named Parsifal [ 26 ], engineered explicitly for systematic reviews across various domains, was utilized to streamline and expedite the systematic review process.

3. Results and Analysis of Results

This section presents the domain’s primary contributions and data analysis while comprehensively describing and examining the gathered information. The bibliometric analysis encompasses descriptive and inferential techniques applied to the articles under study. It is crucial to emphasize that the bibliometric analysis relies exclusively on data from Scopus and Web of Science sources, owing to the constraints imposed by the Bibliometrix library, which necessitates specific metadata for domain analysis [ 19 ]. Regrettably, other databases cannot be utilized for this purpose due to their inability to furnish the requisite metadata. Furthermore, the latter part of this section furnishes responses to the posed inquiries aimed at addressing specific gaps in the existing literature, as identified during the literature review.

3.1. Execution of the Bibliometric Analysis

3.1.1. domain overview.

A bibliometric analysis was performed to assess the utilization of solubilizing microorganisms as a biotechnological method for enhancing the availability of assimilable phosphorus to plants. The studies spanned from 1982 to 2023, totaling 41 years of research data, indicating the maturation of this domain. A comprehensive examination of 495 sources, including journals, books, and other pertinent documents, within the Scopus and Web of Science databases identified 1168 relevant publications, underscoring the extensive literature on this subject.

The analysis unveiled an annual growth rate of 9.31% in the number of documents related to solubilizing microorganisms ( Figure 2 ). This percentage signifies a consistent increase in research activity over time. The papers’ average age was 5.89 years, indicating that research in this field is relatively recent. Moreover, each paper received an average of 26.48 citations, highlighting the research’s influential nature and impact on the scientific community.

General information on the domain of solubilizing bacteria as a biotechnological tool.

Regarding the authors in this domain, the analysis identified contributions from 4253 individuals who have participated in the research, highlighting the collaborative approach within the field. Of the total number of articles analyzed, 32 had a single author, indicating that most articles involved multiple authors, reflecting collaborative efforts in the domain. On average, each article had 4.9 co-authors, further illustrating the collaborative nature of the research. International co-authorships accounted for 14.3% of the total collaborations, suggesting the involvement of researchers from different countries in the study of phosphorus-solubilizing microorganisms as a biotechnological alternative [ 28 ]. Finally, the most common types of documents in this domain are research articles (937) and book chapters (87), indicating a preference for these formats when disseminating research results.

Overall, the data indicate a growing interest in the study of solubilizing microorganisms over time. A strong emphasis on collaborative research has led to a substantial body of literature with significant citation impact.

3.1.2. Annual Scientific Production

The scientific output related to phosphorus-solubilizing microorganisms ( Figure 3 ) has grown substantially in recent years, signifying a heightened interest and activity within the scientific community concerning this field. Initially, during the early 1980s, when the field was still emerging and establishing its fundamentals, the number of published articles per year was relatively low. This was also the time of the “green revolution” [ 29 ], characterized by the indiscriminate use of chemically synthesized fertilizers without considering their environmental and health impacts.

Scientific production in the field of solubilizing bacteria as a biotechnological tool.

Between 1983 and 1990, there was a period of low productivity in research, but from the 1990s onward, there was a gradual rise in scientific output. Since 2006, there has been a consistent and steady increase in published scientific articles, reaching its peak in 2022. This remarkable surge in research activity can be attributed to the ongoing conflict between Ukraine and Russia, as these countries play a significant role as phosphorus fertilizer producers and promoters worldwide. The war has disrupted the supply of these fertilizers, leading to scarcity and, consequently, a growing interest in finding alternative approaches to address crop deficiencies resulting from reduced phosphorus fertilizer availability [ 30 ].

In a longitudinal analysis spanning four decades (1982–2023), 1168 scholarly articles were disseminated. The annual publication count reached a nadir in 1983, 1984, 1986, and 1987, with no articles published. Conversely, the zenith was attained in the antecedent year, 2022, marking the highest annual output.

The growing interest and research in solubilizing microorganisms since the 2000s have been driven by their potential biotechnological applications, benefits to soil health, and minimal environmental impact [ 31 ]. The field has seen exponential publication growth since 2018, indicating its maturation and significant impact on agriculture and environmental domains. The surge in research is also linked to the critical role of phosphorus, an essential macronutrient in plant growth, and its scarcity [ 32 ]. Given the increasing demand for food and sustainable agriculture, solubilizing microorganisms are being explored as a viable, efficient, cost-effective, and eco-friendly biofertilizer solution to address phosphorus limitations in crop production [ 33 ].

3.1.3. Relevant Sources in the Domain

This section presents a comprehensive scientific and technical analysis of the top ten essential sources focused on the biotechnological utilization of solubilizing microorganisms to enhance phosphorus availability. Figure 4 displays valuable information regarding the number of articles published per source, their corresponding H-index, and CiteScore.

Primary sources in the domain.

The H-index is a measure used to assess a journal’s citation productivity and impact. It considers the number of articles that have received at least the same number of citations [ 34 ]. On the other hand, CiteScore is a metric that represents the average number of citations per article derived from the Scopus database. The CiteScore is divided into quartiles, with Q1 indicating the highest level of influence and Q4 indicating the lowest [ 35 ].

The bibliometric analysis reveals that the journal Science of the Total Environment has the highest number of articles published in the field, with 43 articles accounting for 19% of the top 10 sources. Additionally, this journal boasts an impressive H-index of 317 and is placed in the Q1 quartile, signifying its position as the most influential and reputable journal in this research domain. Similarly, other high-impact journals, such as the Journal of Environmental Quality , Frontiers in Microbiology , Frontiers in Plant Science , the Journal of Environmental Management , and Plant and Soil also exhibit noteworthy H-index values above 180. These journals are placed in the Q1 quartile based on their CiteScore, further solidifying their impact and reputation within the field. In contrast, the Journal of Soil Science and Plant Nutrition possesses a comparatively lower H-index of 53 and is ranked in the Q1 quartile, indicating its relatively lower influence and reputation in this domain.

The ten journals contribute 223 articles, representing approximately 20% of the overall publications within this specific research domain. This finding underscores the significance of scientific production in high-impact sources and highlights the importance of research conducted in the exploration and biotechnological application of phosphorus-solubilizing microorganisms. This area emerges as novel and of great relevance within the scientific community.

Metrics of the Leading Journals in the Domain

In the analysis of the critical metrics of the most significant sources in the domain ( Figure 5 ), it is observed that Science of the Total Environment and the Journal of Environmental Quality have considerable influence, as they have high H-indexes and occupy the top positions in terms of total citations and published articles. Similarly, Frontiers in Microbiology and Frontiers in Plant Science stand out as prominent sources due to their substantial m-indexes, which indicate the rapid attainment of high H-indexes soon after their initial publications. Despite being the newest in the table, Microorganisms is the most promising source in the domain, as it has the highest m-index and ranks high on a g-index, despite being the newest source in the table. Likewise, Communications in Soil Science and Plant Analysis and the Journal of Plant Nutrition are considered the least influential and impactful sources within this domain, as evidenced by their lowest H, g, and m indices, total citations, and published articles.

Primary sources’ productivity metrics in the domain.

Furthermore, it is imperative to acknowledge the inverse relationship between the H-index and the quantity of published articles. Entities with elevated H-indexes typically possess fewer articles, signifying a prioritization of quality over quantity. Additionally, a correlation exists between the total number of citations and the year of initial publication, suggesting that sources accrue citations over time, thereby establishing their significance within the respective field as they mature.

3.1.4. Relevant Universities in the Domain

This study reveals the publication trends in using solubilizing bacteria to enhance plant phosphorus assimilation ( Figure 6 ). Zhejiang University leads with 47 publications, while Universiti Putra Malaysia has the fewest at 22. The leading universities are predominantly in China, Pakistan, India, Australia, and Malaysia.

Central universities contributing the most to solubilizing bacteria as a biotechnology approach.

In terms of country distribution, China has the highest number of universities (four) engaged in this research, followed by India (three), Pakistan, Australia, and Malaysia (one each). China also leads in the total number of publications (114), followed by India (80), Pakistan (37), Australia (27), and Malaysia (22).

The data suggest that using solubilizing bacteria as a biotechnological strategy to improve phosphorus assimilation in plants is a growing research trend, especially in Asian countries. This could be due to the limited availability of chemically synthesized phosphorus fertilizers, prompting the search for alternative solutions [ 36 ]. However, other factors, such as funding, infrastructure, policies, and each country’s specific context, may also influence research productivity and its correlation with national agricultural development and production models [ 37 ].

Turning to the scientific production of the leading universities in the domain ( Figure 7 ), a noteworthy trend has been observed since 2009. There has been a steady increase in scientific production from these universities, indicating the emergence of a novel topic that initially received limited attention from the research community. Zhejiang University stands out with the highest research output among all relevant universities. Its research output has consistently remained high compared to others since 2010, showcasing its intense focus and significant contributions to the field. Conversely, the Faisalabad Agriculture University exhibits relatively lower research output compared to other universities, but there has been a gradual increase since 2019, suggesting a growing emphasis on this aspect.

Chronological evolution of the research production of the relevant universities in the domain.

Along this line, Zhejiang University is the primary contributor to research output on solubilizing bacteria as a biotechnological approach to improving plant phosphorus assimilability. Adelaide University and the Faisalabad University of Agriculture demonstrate different levels of involvement and increasing research output in this field. On the other hand, the Indian Council of Agricultural Research and Eternal University have limited or no research output in this area.

3.1.5. Scientific Production of the Countries in the Domain

The global output of research on PSB, with the leading contributors being India, China, Brazil, Pakistan, and the United States, accounts for 52.6% of the total publications. These countries’ significant involvement is attributed to their large populations, extensive agricultural activities, and phosphorus-rich soils. The study of solubilizing bacteria is particularly relevant in these regions due to their extensive agricultural sectors and high demand for phosphorus fertilizers. As a result, these nations show a strong inclination towards adopting biotechnological alternatives to enhance plant growth and nutrient cycling [ 38 ].

The research productivity of various countries ( Figure 8 ), including the United States, Australia, Japan, Morocco, Germany, and Egypt, is also highlighted. These countries, despite their diverse geographical, climatic, and economic conditions, demonstrate significant engagement in studying phosphorus-solubilizing bacteria, indicating their adaptability and versatility as practical biotechnological tools [ 39 ].

Scientific production is segmented by country in the domain.

A comparative analysis reveals that Asia has the highest scientific yield in solubilizing bacteria research, reflecting the active participation of Asian nations in this domain. However, the contribution from American nations, such as Brazil, the United States, Mexico, Canada, and Colombia, is comparatively lower when juxtaposed with Asia, Europe, and other regions.

This paper also notes a substantial disparity between the most and least productive countries, with some regions needing more publications in this field. This discrepancy may be due to inadequate resources, infrastructural limitations, insufficient expertise, or lack of awareness of the potential benefits of solubilizing bacteria. The agricultural production patterns in these regions do not prioritize or necessitate extensive research on solubilizing bacteria.

Collaborations in the Domain Segmented by Country

In terms of the distribution of research publications on using solubilizing bacteria to enhance plant phosphorus assimilation [ 40 ], Figure 9 shows that these publications are from single countries, with cross-country collaborations accounting for only 15.3% of total publications. This suggests a low level of international collaboration in this research domain. Countries like India, China, the United States, Brazil, and Pakistan have many publications authored solely by their nationals, ranging from 75% to 92%. However, Japan and Spain, among the top 10 most productive countries, have at least 30% of their publications resulting from international collaborations.

Distribution of collaborations among authors in the domain.

Figure 9 also shows that there is potential for improving international collaboration in this field, given the value of the use of solubilizing bacteria as a biotechnological solution, especially in regions characterized by low soil fertility and significant phosphorus deficiency, which have a large niche for the development of this type of research. Therefore, multinational publications can facilitate the exchange of knowledge, resources, infrastructure, and expertise, which vitally contribute to improving the quality and impact of research [ 41 ]. It should be noted that the limited level of international collaboration observed in this domain can be attributed to several factors, such as language barriers, lack of funding opportunities, divergent research priorities and policies, and restricted access to data and resources.

Cross-Country Collaboration in the Domain

This section quantitatively examines the frequency of international collaborations among researchers utilizing solubilizing bacteria as a biotechnological approach to enhance plant phosphorus assimilation. The significance of this lies in the ability of these microorganisms to catalyze the transformation of insoluble phosphorus compounds into soluble forms, thereby facilitating their absorption by plants. Figure 10 provides a visual representation of the foremost collaborations, where the red lines indicate the relationship between countries, showing that the most recurrent collaborations are between China and Pakistan (14 instances), China and the United States (9 instances), China and Germany (6 instances), and India and Saudi Arabia (6 instances). Furthermore, China emerges as the most prolific collaborator globally, with 34 instances of multi-country collaboration. India is a close second, with 31 collaborations involving countries like Saudi Arabia and China. The United States is ranked third, with 27 collaborations spanning various countries globally, followed by Pakistan in the fourth position, with 21 collaborations.

Productive research relationships between different countries in the domain.

The geographic distribution of research collaborations focuses on Asia, which has extensive collaborations with Europe and North America. In contrast, Africa, South America, and Oceania have fewer collaborations, with South America and Asia’s collaborations being particularly infrequent due to geographical, cultural, and language barriers [ 42 ].

Specific collaborations are highlighted, such as the frequent collaborations between China and Pakistan due to geographical proximity, historical ties, political alignment, and shared agricultural interests [ 43 , 44 ]. Similarly, collaborations between Brazil and Germany are significant in biotechnology and environmental sciences, while collaborations between China and Germany reflect their solid economic relationship and mutual interest in innovation and sustainability [ 45 ].

In Africa, intra-regional cooperation needs improvement due to limited resources for conducting and publishing research and inadequate access to international research networks [ 46 ].

Finally, there is the potential for increasing research synergies and inter-regional cooperation, particularly between regions at different development stages. This collaboration fosters the reciprocal transfer of knowledge and technology, specifically using solubilizing bacteria to enhance phosphorus bioavailability and promote plant growth.

3.1.6. Domain Funding Sources

This section presents a comprehensive scientific analysis of the primary financial entities that enable the implementation of solubilizing bacteria as a biotechnological approach to enhance plant phosphorus uptake, as illustrated in Figure 11 . The leading ten entities include four Brazilian institutions, two Chinese institutions, three Indian institutions, and one Canadian institution. The National Natural Science Foundation of China is the most significant contributor, supporting 59 academic papers. It is closely followed by Brazil’s Conselho Nacional de Desenvolvimento Científico e Tecnológico, which has sponsored 29 papers, and Brazil’s Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, endorsing 24 papers. These findings highlight the significance of employing solubilizing bacteria to amplify phosphorus uptake across different countries, especially those with the highest publications in this domain. As a result, the research efforts of these nations are primarily focused on expanding knowledge in this field. Moreover, the funding sources include national bodies and regional foundations, emphasizing the broad acknowledgment of the research’s significance and potential influence within each respective country at national and regional scales.

Main research sponsors in the field of solubilizing bacteria as a biotechnological alternative.

In this regard, greater collaboration and communication between the different funding sources, sponsoring countries, and countries with fewer resources could be encouraged to improve the domain. This facilitates the exchange of knowledge, experiences, and resources on this topic and the identification of common challenges and opportunities for innovation. In addition, greater attention and support should be given to underrepresented countries or regions with the potential or need to use solubilizing bacteria as a biotechnological strategy to improve phosphorus assimilation in plants, such as Colombia or other Latin American countries.

It is also essential to indicate that more research should be conducted to explore the mechanisms, diversity, efficiency, and application of solubilizing bacteria in different soil types, crops, climates, and management practices. In addition, research is warranted to investigate this strategy’s environmental and socioeconomic impacts. These research efforts would contribute to a deeper understanding of the subject and pave the way for advances in this field.

3.1.7. Conceptual Structure of the Domain

A specific research field’s theoretical framework is constructed to facilitate the understanding and investigate the interrelatedness of diverse concepts within that domain [ 47 ]. In this context, the emphasis is on examining solubilizing bacteria and their contribution to the augmentation of phosphorus uptake by plants via biotechnological methodologies.

The initial step involves a quantitative assessment of the domain’s predominant terminology. This involves thoroughly examining the most frequently utilized keywords in scholarly articles and those indexed in scientific databases. This analysis is crucial, as it identifies any discrepancies between the terminologies proposed by the authors and those algorithmically suggested by the databases.

A keyword frequency analysis of the scientific literature ( Figure 12 ) related to using solubilizing bacteria to enhance plant phosphorus assimilation was conducted. The most frequently used keywords are “phosphorus” and “biofertilizer”, indicating a strong interest in improving phosphorus availability using microbial inoculants. Other prevalent keywords include “phosphate solubilization”, referring to the process of converting insoluble phosphorus into plant-assimilable forms, and “PGPR” (plant-growth-promoting rhizobacteria), a type of root zone that enhances plant growth and yield. The terms “plant growth promotion” and “sustainable agriculture” frequently appear, underscoring the literature’s objectives of enhancing plant productivity and quality through biotechnological approaches [ 48 ].

Thematic analysis of the main keywords used in the domain.

Regarding the keywords indexed by scientific databases, “phosphorus” remains the most prevalent term, aligning with the authors’ keywords and indicating its significant importance as per the databases’ algorithms [ 49 ]. A new keyword emerging from the database analysis is “soil”, which appears 267 times [ 38 ]. This previously unmentioned keyword suggests that soil is the primary medium for phosphorus solubilization and plant uptake. The third and fourth most frequently used keywords are “fertilizers” and “phosphate”, occurring 231 times. Together, these results imply a considerable demand in the literature to explore alternative and sustainable methods for improving the processes of phosphorus availability for plant or crop uptake.

Considering the above, the domain of study is well-defined, with a consistent focus on investigating the role of bacteria in facilitating phosphorus uptake. The central theme within this domain is the influence of phosphorus on plant growth, while other factors remain subjects for further examination.

Clustering According to the Thematic Association of the Domain

This section examines the thematic distribution of the domain, which refers to a visual representation showing the occurrence of specific words within the domain in the metadata of the analyzed documents. Topic maps classify these occurrences into four categories: driving, niche, emerging, and core topics. This classification is based on the concentration and importance of the topics [ 50 ]. Figure 13 illustrates the distribution of themes within the domain using a clustering technique that captures their associations.

Thematic clustering of the domain for solubilizing bacteria as a biotechnological alternative.

In considering the primary themes of the domain, it is essential to highlight phosphorus as a central element of agriculture. Phosphorus is crucial in promoting plant growth, consistent with its essential nature for plant development [ 51 ]. However, the availability of phosphorus is limited due to its reactivity with other elements. To address this limitation, using solubilizing microorganisms can improve plant access to phosphorus, thus directly improving the agricultural process [ 52 ].

A significant and relevant relationship observed in the domain is the struvite precipitation process, which is closely related to the fundamental issues of this field; among the most crucial precipitation techniques are thermodynamic and kinetic modeling [ 53 ] and chemical precipitation [ 54 ]. Struvite, a mineral widely discussed in the scientific literature, is crucial for recovering nitrogen and phosphorus from anaerobic digestate [ 55 ]. Therefore, struvite precipitation offers an exciting method to recover phosphorus sustainably from sources like manure from various animals [ 56 ].

Furthermore, a correlation has been observed between work focusing on eutrophication, fertilization, and phosphorus availability. This has to do with the process of phosphorus extraction, and its subsequent transport in fertilizers, crops, and other products can lead to the accumulation of this mineral in certain soils around the globe. This accumulation generates elevated levels of phosphorus, which increases the likelihood of phosphorus runoff into aquatic ecosystems through erosion in lakes, streams, and rivers [ 57 ]. Excessive enrichment of nutrients and minerals, such as phosphorus, can cause problems, such as uncontrolled algal growth, thus negatively affecting aquatic tributaries and their ecosystems [ 58 ]. Therefore, managing these processes by using inorganic nutrients is essential in this field.

Another notable relationship among the various papers concerns PGPR plants. These bacteria are essential in sustainable agriculture, as they enhance plant growth through biological nitrogen fixation, phytohormone production, and phosphate solubilization [ 59 ]. Therefore, PGPRs constitute an attractive biofertilizer option that can contribute to the advancement of more sustainable agricultural practices.

The motor themes are well-developed and essential for structuring a research field [ 60 ]. Figure 13 indicates that the presence of motor themes is limited, indicating the domain’s novelty. More importantly, it highlights the need to establish solid conceptual foundations in this domain, which are consistently employed in various studies to avoid redundancy and thus incentivize advancement.

Niche topics refer to very specialized aspects of research in the field of phosphorus-solubilizing bacteria as a biotechnological alternative to improve plant assimilation [ 61 ]. One such topic focuses on the utilization of phosphorus to improve the overall agronomic efficiency of crops, which can be influenced by several factors, such as the solubility of phosphorus in water and citrate, the chemical composition of water-soluble solid phosphorus fertilizers, the various forms of liquid fertilizers, and the chemical reactions that occur when phosphorus fertilizers are applied to the soil [ 62 ].

Another niche topic within this field revolves around superphosphates, which offer a significant advantage as phosphorus-rich fertilizers. These fertilizers are used to replenish soil nutrient levels and promote plant growth. They also facilitate rapid root formation and growth, allowing plants to resist water shortages, especially in areas where availability is limited [ 63 ]. Another interesting relationship is between gluconic acid, in which PSB can convert insoluble phosphorus to soluble phosphorus, thus improving plants’ uptake and use of soil phosphorus. Of all organic acids, gluconic acid is the most frequent solubilizing agent for mineral phosphate.

As discussed in several articles, a vital connection observed in this field concerns the interaction between arsenic and groundwater. Excessive use of conventional phosphorus fertilizers, known for their ability to increase agricultural productivity, can lead to water pollution through eutrophication [ 17 ]. This phenomenon can lead to the accumulation of harmful substances in the soil, such as selenium and arsenic, thus affecting their concentration levels. There is another association between gluconic acid and PSB. These bacteria can convert insoluble phosphorus into a soluble form, thus facilitating plants’ uptake and utilization of phosphorus from the soil. Among all organic acids, gluconic acid is the most common mineral phosphate-solubilizing agent [ 64 ]. Another niche topic in the domain is the utilization of manure as a source of organic phosphorus. This makes possible the use of phosphorus-solubilizing bacteria, which are microorganisms that can convert insoluble phosphorus into a soluble form that plants can absorb. Although some advances are documented in the scientific literature, these have yet to be widely reported in this field of study [ 38 , 64 ].

Thematic Evolution of the Domain

Thematic evolution analysis is a systematic methodology utilized to identify, quantify, and visually represent the progression of a specific area of research. It achieves this through the implementation of clustering techniques and co-word network analysis. Cobo initially proposed this analytical approach [ 65 ]. A comprehensive examination of the scientific literature was conducted for the analysis, covering distinct periods: 1982 to 2012, 2013 to 2015, 2016 to 2020, and 2021 to 2023 ( Figure 14 ). The division of these time frames was purposeful, aiming to reveal emerging trends within the field of study and to gather insights into future expectations. The core methodology employed to investigate thematic evolution involves the construction of concurrence networks. These networks categorize and interconnect frequently used words within scientific articles.

Thematic evolution of the domain of solubilizing bacteria as a biotechnological alternative.

Substantial advancements were made in establishing the foundational principles of this research domain between 1982 and 2012. Several key terms were identified as forming the bedrock of this area, including nutrients, natural phosphate, plant growth promotion, soil, phosphorus, phosphate, and apatite [ 66 ]. These terms are of fundamental significance concerning phosphorus assimilation and closely aligned with the initial stages of research exploration.

During this early period, researchers likely concentrated on exploring the potential of specific bacteria to enhance the solubility of insoluble phosphorus compounds prevalent in soil, such as rock phosphate and apatite. Apatite, a phosphate mineral and the primary form of natural phosphate, is vital in determining phosphorus availability to plants, thereby promoting their growth and productivity [ 67 ]. Additionally, investigations into the interactions between microorganisms, soil properties, nutrient dynamics, and plant responses were likely undertaken about these bacteria.

Overall, the knowledge base established during this phase forms the groundwork for subsequent topics that will emerge as integral components of this research domain.

From 2013 to 2015, several terms, such as plant growth promotion, agriculture, biofertilizers, plant-growth-promoting bacteria, fertilizers, phosphorus, struvite, microbial biomass, and nutrients, constantly appeared in the scientific literature. Research during this period focused on investigating the potential of plant-growth-promoting bacteria, specifically in the form of biofertilizers. This term describes organic fertilizers derived from organisms that aim to improve nutrient availability and promote plant growth.

The mention of struvite indicates the scientific community’s interest in understanding the role of soil microbial biomass and phosphorus sources, such as struvite, in improving phosphorus assimilation [ 66 ]. The recurrent use of these terms in different articles suggests that the scientific community is actively studying how plant-growth-promoting bacteria improve phosphorus assimilation in plants. The goal is to develop efficient and sustainable agricultural practices while addressing the challenges associated with plant growth and agroecosystem sustainability.

When analyzing the relationship between these elements, it becomes evident that the term “plant growth promotion” has been constantly emphasized over the years in different research worldwide. In addition, it should be noted that minerals, such as apatite in the previous period and the use of struvite, speak to the concern to find new sources of soluble phosphorus to make up for the deficiencies of this element in the soil to avoid a loss of productivity in agricultural systems [ 68 ].

According to the analysis of the scientific literature published between 2016 and 2020, researchers have undertaken research on solubilizing bacteria to improve phosphorus assimilation in plants to understand which are the most efficient biochemical mechanisms in the transformation of insoluble phosphorus compounds into soluble forms, thus increasing its availability in the soil and, ultimately, avoiding a deficit of this element that allows for the correct growth of plants or crops [ 69 ]. The scientific community has explored various mechanisms related to phosphate utilization and investigated the potential advantages associated with the use of rhizobacteria and other types of bacteria that promote plant growth. An exciting development in this field is the emergence of nitrogen as a crucial new research element, reflecting the evaluation of hydrogen to understand the overall nutrient balance and its impact on plant growth. In addition, researchers have continued to emphasize the importance of minerals, such as struvite, as a promising source of phosphorus while expressing growing concern about the potential adverse effects of eutrophication. This concern has led to research into methods to promote efficient phosphorus uptake in plants.

The scientific literature between 2016 and 2020 suggests that using solubilizing bacteria as a biotechnological tool can improve plant phosphorus assimilability. These bacteria can solubilize phosphates by producing organic acids, such as citric and gluconic acids. In addition, they may have other PGPR properties, such as the production of Indole-3-Acetic Acid (IAA), a phytohormone that stimulates root growth. The composition of the bacterial community and the interactions between bacteria, fungi (such as Aspergillus niger ), and the plant microbiome are crucial factors influencing the efficacy of solubilizing bacteria in sustainable agriculture [ 70 ]. Solubilizing bacteria can interact with other microorganisms and form biofilms in the soil, affecting the availability and cycling of other nutrients, such as nitrogen, by reducing dependence on chemical fertilizers and improving nutrient use efficiency. Applying solubilizing bacteria as biofertilizers holds promise for improving the productivity and sustainability of agroecosystems.

In detail, it can be found that one mechanism used by solubilizing bacteria to solubilize phosphorus is the secretion of organic acids, such as gluconic acid, citric acid, and oxalic acid. These organic acids can lower the soil pH and dissolve insoluble phosphates, such as hydroxyapatite and struvite, into soluble forms [ 32 ]. Hydroxyapatite and struvite are two common mineral forms of phosphorus in soils and wastewater. They are also potential sources of phosphorus recovery and recycling as fertilizers.

3.2. Execution of the Literature Review

The following is a solution to the above research questions to complement the domain analysis on using solubilizing bacteria as a biotechnological element to improve plant phosphorus assimilation.

3.2.1. What Is the Importance of Phosphorus in Plant Physiology or Nutrition?

Phosphorus is a crucial macronutrient necessary for plant growth and function. It plays a fundamental role in several biological processes, such as the development of roots, grains, and flowers. Phosphorus is also involved in forming cell membrane phospholipids and genetic material. In addition, it is a vital component in photosynthesis, glycolysis, and fatty acid synthesis [ 71 ].

Despite its importance, the low availability of phosphorus in the earth’s crust limits agricultural production due to its low mobility in the soil, preventing the plant from absorbing it from the soil solution and having the capacity to grow without deficiencies of this element. For example, phosphorus deficiency can affect the biochemical reactions of the cell’s energy metabolism as it is an integral part of the molecules that accumulate energy as ATP. It is a result of photosynthesis and used in plant respiration. Therefore, generating new cells to produce roots at the beginning of vegetative cycles is vital. Compounds analogous to ATP, such as Uracil triphosphate, Guanosine triphosphate, and Cytosine triphosphate, are required to synthesize sugars, phospholipids, and ribonucleic acids [ 72 ]. Phosphorus is found in the soil pool in different forms, including oxygen-bound phosphates in rocks, which are diluted during weathering, allowing plant roots to absorb more quickly in organic and inorganic forms [ 73 ]. New alternatives are currently being sought to improve phosphorus uptake and take advantage of phosphorus sources present in the soil to reduce dependence on exogenous fertilization in agricultural production [ 33 ].

The phosphorus cycle in the soil is a complex and dynamic system that involves the accumulation of this element in the microbial biomass of phosphorus solubilizers, which include four general forms of phosphorus: available inorganic, organic, adsorbed, and as a primary mineral [ 6 ]. These different forms of phosphorus are recycled through water by the earth’s crust and living organisms in a sedimentary process characterized by the slow migration of phosphorus from deposits on land to marine sediments and its return to the soil and ocean. P is a fundamental component in the energy transduction processes, nucleic acid synthesis, and the creation of essential biomolecules. It is integral to the metabolic and physiological functions of plants. The presence of phosphorus promotes the development of a robust root system, facilitates appropriate flowering, and ensures efficient seed production. These factors are critical in agricultural practices to guarantee optimal crop yields. Phosphorus aids in accelerating maturation and enhancing stress resistance, attributes that improve the efficiency and sustainability of crop cultivation. Therefore, the effective management of phosphorus in agricultural practices is vital for maintaining plant health and optimizing the productivity and sustainability of our agricultural systems [ 33 ].

3.2.2. What Are the Sources of Phosphorus Available in the World?

In the soil, plants can access two main types of phosphorus: inorganic phosphates and organic phosphates. However, plants are unable to absorb insoluble inorganic phosphorus compounds. The pH level of the soil influences the availability of inorganic phosphorus. Some insoluble inorganic phosphorus compounds include apatite, oxyapatite, struvite, and hydroxyapatite. Under favorable conditions, these compounds can dissolve and become accessible for plants to utilize [ 74 ]. Phosphate anions (PO 4 3− ) are highly reactive and can be immobile through precipitation when combined with metal ions in the soil. In acidic soils, a substantial amount of phosphorus in fertilizer reacts with cations like Al 3+ or Fe 3+ through a precipitation reaction. This can result in the formation of orthophosphate ions, which plants can take up. Additionally, certain bacteria in the soil play a vital role in the process of solubilizing phosphorus, converting it into forms that plants can assimilate [ 33 ].

Phosphorus concentrations in soil solutions typically range from 0.1 to 1.1 mg·L −1 . Among these concentrations, more than half may exist as soluble organic components released by dead cells or as colloidal organic components. Phosphate fertilizers are a type of simple fertilizer obtained from phosphate rock, a set of natural minerals containing a high concentration of P compounds. Phosphate rock is the primary raw material for producing this type of fertilizer, which does not generate environmental pollution like chemically synthesized fertilizers [ 75 , 76 ]. The P bioavailability in the pedosphere is contingent upon specific determinants, such as the soil’s pH value and particular bacterial taxa that facilitate the solubilization process, thereby rendering P accessible for plant absorption. Although fertilizers supplement the soil with phosphorus, the assimilation of this macronutrient may be constrained by the soil’s physicochemical properties. Conversely, phosphate fertilizers derived from phosphate rock represent a sustainable alternative due to their negligible contribution to environmental contamination [ 77 ].

Phosphate Rock

Phosphate rock deposits are composed of arenites with a diphosphorus pentoxide (P2O5) content ranging from 20% to 30% [ 76 ]. Soil fertility is limited by certain properties, such as pH; acid soils with high levels of Fe +3 and Al +3 cations have low plant-available phosphorus content and tend to fix phosphorus applied as phosphorus fertilizer. Consequently, phosphorus applied as phosphorus fertilizer is fixed in these soils, which reduces the effectiveness of water-soluble phosphate fertilizers, such as triple superphosphate and diammonium phosphate. In such cases, unprocessed phosphate rock presents an attractive alternative. RF contains calcium carbonates, which help achieve near-neutral soil pH levels. This, in turn, improves the availability of soluble phosphorus in acid soils, such as andosols and oxisols [ 78 ]. Conversely, limited solubility poses a significant constraint on phosphorus sources derived from phosphorite rocks, as corroborated by numerous studies conducted across various Latin American nations [ 79 ]. These investigations have demonstrated that the outcomes are not consistently favorable when phosphorite rocks are directly applied to the soil, either in their intact or pulverized state, particularly in short-duration crops and in soils with alkaline pH, where phosphorus tends to be sequestered by soil colloids [ 80 ]. However, finely pulverized phosphorite rock may gradually convert its phosphorus to a soluble form in the soil. This process is influenced by soil acidity and phosphorus-solubilizing microorganisms, which vary among different soil types. This is essential to augmenting phosphorus availability and ensuring adequate crop nutrition, both short-cycle and perennial [ 81 ].

Phosphorus Availability in Plants

Phosphorus is an essential element for the growth and development of plants and microorganisms, as it plays a crucial role in the mixture of energy accumulation and release processes during cell metabolism. However, it is essential to consider that soluble phosphorus is limited in agricultural production and natural ecosystems [ 78 , 82 ]. Moreover, its availability depends on the soil type, as approximately 50 to 60% is found in an organic fraction, while the rest is in inorganic form [ 83 ]. Plants take up soluble inorganic phosphorus, but when introduced into the soil in percentages above 90%, it is rapidly converted into forms unavailable to plants [ 84 ]. Therefore, much of the applied soluble phosphate fertilizers are not used by plants but are stored in the soil or become sources of pollution of water bodies [ 85 ].

It is important to note that phosphorus is found in the soil in non-nutritious forms for plants, which must assimilate the element from the soil even though it is found in low concentrations [ 84 ]. Typically, phosphorus levels in soil vary between 5 and 30 mg kg −1 , representing low nutrient ratios. In alkaline soils, phosphorus reacts with cations, such as Ca +2 , Fe +3 , and Al +3 , which causes its precipitation or fixation, decreasing its availability to meet the needs of plants or crops [ 86 ]. On the other hand, it should be mentioned that phosphorus fertilization in agricultural systems significantly increases production costs [ 87 ]. For this reason, several studies focus on the use of the reserves that the soil has in non-assimilable forms to transform them into usable forms; one of the alternatives is the use of microorganisms called phosphorus solubilizers. Their mechanisms of action between plant microorganisms have to be an effective relationship so that the availability of phosphorus has positive effects on crops [ 86 , 88 ].

3.2.3. How Is Phosphorus Availability Improved Using Phosphorus-Solubilizing Microorganisms?

The escalating requirement for sustenance, propelled by demographic augmentation and escalating affluence, necessitates amplified crop yields. Regrettably, anthropogenic interference has precipitously diminished the fecundity of agrarian soils, particularly P, through overutilization, thereby jeopardizing production and augmenting the global risk of famine [ 89 ]. P is indispensable, yet its limited bioavailability poses a challenge in over 40% of global soils, exacerbating this issue through overexploitation due to the intensification of agricultural production systems. Without corrective measures, phosphorus reserves could be exhausted within a few decades.

Phosphate fertilizers address P deficiencies in crops, but plants can only utilize between 5% and 25% of their content. Upon interaction with certain elements, such as Ca 2+ , Al 3+ , and Fe 3+ , and acidic pH, these compounds rapidly become inaccessible to plants, which is where PBS can intervene. These microorganisms enhance P bioavailability in the soil and promote plant growth, representing a sustainable and efficacious solution [ 90 ]. Despite the remarkable potential of PBS, it is crucial to comprehend its impact on plant nutrition. These bacteria can capture phosphorus retained in the soil and convert it into soluble forms [ 74 ]. They accomplish this by releasing various organic acids into the soil, facilitating phosphorus solubility and its availability to plants (as will be elaborated in subsequent sections).

Phosphorus-solubilizing microorganisms are capable of releasing phosphorus into soils through biological solubilization mechanisms. This process promotes plant growth and has been extensively documented in the scientific literature, with phosphate release mediated by phosphorus-solubilizing microorganisms being one of the most detailed mechanisms [ 86 ]. PSB microorganisms are concentrated in the rhizosphere and metabolically active in the zone near or inside of the root [ 91 ]. Typically, amounts ranging from 10 1 to 10 10 CFU/g soil can exceed 2000 kg ha −1 of microbial biomass. Bacteria are exceptionally diverse in this ecosystem, with a wide range of genera, such as bacilli, cocci, and spirochetes, among others [ 92 ]. PSB communities are ubiquitous, with variations in forms and populations in different soils, and depend on the physicochemical properties of phosphorus content and cultural activities of agricultural systems. In addition, it has been shown that herbaceous plants present more abundant PSB communities than perennial plants [ 93 ]. It has been reported that these communities increase their populations in agricultural systems, especially in soils cultivated with cereals [ 94 ].

Soluble organic phosphorus is mineralized by nonspecific extracellular phosphatases in PSB microorganisms [ 95 ]. These microorganisms can break phosphodiester or phosphoanhydride bonds present in organic matter. In addition, there are specific phosphatases, such as phytases, whose role is to release phosphorus from phytic acid, one of the primary forms of organic phosphorus reserves in the soil [ 96 ]. The plant recruits PSB microorganisms in the rhizosphere zone, and part of them enters the root tissue. This complex biological interaction allows plants to have sources of soluble phosphorus from organic matter or inorganic substances while providing them with organic substances (secondary metabolites) for their survival [ 97 ].

In P solubilization and mobilization, PSB microorganisms play a fundamental role as they are responsible for solubilizing about 40% of the total phosphorus in the soil. The taxonomic genera that stand out are Bacillus , Pseudomonas , Enterobacterales , Burkholderia , and Rhizobium [ 98 , 99 ]. Phosphorus solubilization and mobilization by microorganisms is an indispensable mechanism for promoting plant growth, as numerous studies have shown that plant responses to inoculation by these microorganisms contribute to nutrition and improve agricultural systems [ 76 , 100 ]. The groups of microorganisms that stand out for their ability to solubilize phosphorus are shown in Table 3 .

Outstanding groups of microorganisms for phosphorus solubilization.

Microorganisms present in the soil play an active role in mineralizing organic phosphate. Laboratory studies have shown that microorganisms of the PSB group utilize various sources of organic phosphorus to meet plant needs and can also accelerate the incorporation of this element into the soil solution for storage [ 114 ]. Microbial activity plays a crucial role in transforming organic matter in the soil. However, the amount of mineralized phosphorus and its relative contribution to plant nutrition in agricultural systems still need to be fully understood due to the need for more research in this field [ 115 ]. Some microorganisms of the PSB group utilize phosphorus-rich organic substances, such as phytates, for phosphorus solubilization and mineralization, making them assimilable by plants [ 116 ]. Phosphorus availability for plant nutrition can be improved by inoculating microorganisms producing phytase enzymes or through exogenous addition of phytases to roots, especially when phytate is present in the substrate [ 116 , 117 ]. However, a more significant response to inoculation has been observed in plants with large amounts of phytate.

The solubilization of inorganic phosphorus produces and releases to the medium siderophores, hydroxyl ions, and organic acids, such as acetic, gluconic, oxalic, succinic, and malic acids, among others. These acids of low molecular weight, in addition to lowering the pH of the medium, act as fundamental chelators for the solubilization of inorganic phosphorus, which forms complexes with Ca +2 , Fe +3 , and Al +3 cations. Several microbial groups produce enzymes, such as phosphatases, C-phosphorus lyases, and phytases, to mineralize organic phosphorus, releasing different forms of organic phosphorus into the soil solution through hydrolysis of phosphodiester bonds [ 118 ].

In general terms, phosphorus availability in the soil for plant nutrition is regulated by three main processes that affect its concentration in the soil solution: dissolution/precipitation, adsorption/desorption, and interactions between phosphorus in solution and soil solid surfaces and mineralization/immobilization, which are biologically mediated conversions between organic and inorganic forms of phosphorus [ 119 ].

Inorganic phosphorus is present in the soil solution as orthophosphate anions, coming from the mineralization of organic materials and the solubilization of mineral sources [ 118 ], whose concentration varies over time due to environmental conditions and the availability of organic matter and parent material that is rich in inorganic phosphorus. The concentration of orthophosphate anions in the soil solution ranges from 200 to 5000 mg phosphorus kg −1 [ 115 , 118 ].

Finally, mycorrhizal symbiosis is an ancient mutualistic relationship between fungi of the order Glomales (Zygomycetes) and the root systems of most plant species. This symbiotic relationship, which dates back some 398 million years, predates the symbiotic interactions of plants in the family Fabaceae [ 120 ]. It is estimated that about 90% of all terrestrial plant species participate in this symbiosis, which plays a crucial role in the functioning of global natural ecosystems [ 121 ]. In this symbiotic relationship, plants provide fungi with carbon and secondary metabolites derived from their photosynthates. In return, the fungi enhance the plants’ ability to absorb phosphates and other minerals from the soil and produce phytohormones that increase the root surface area and strengthen resistance against soil pathogens [ 122 ].

A significant challenge for mycorrhizologists is deciphering the signaling mechanisms that enable efficient colonization of host plants by these fungi. Several methodologies have been developed to study these interactions and cultivate mycorrhizal fungi under laboratory conditions. These include traditional soil-based systems, aeroponic and hydroponic methods, and the recent in vitro root organ culture technique [ 122 ]. It is known that about 150 species of mycorrhizal fungi can colonize approximately 225,000 plant species. This broad colonization capacity indicates that arbuscular mycorrhizal fungi are not limited to symbiotic relationships with a single plant family, unlike in the Fabaceae family [ 123 ]. The high adaptability of these fungi and their integration into many plant species support the use of mycorrhizal-fungi-based biotechnologies to improve agricultural practices and manage crops in diverse plant environments, highlighting the importance of symbiotic relationships to promote sustainable agricultural practices [ 120 ].

To mitigate low P availability, plants developed different strategies, such as adaptive biochemical and symbiotic mechanisms, to increase inorganic phosphorus acquisition and improve the efficiency of its internal utilization [ 115 , 124 ]. One of the most important is the biological association of the root with free-living soil microorganisms capable of solubilizing organic and inorganic phosphorus sources, respectively [ 125 ].

3.2.4. What Enzymes or Mechanisms Do Phosphorus-Solubilizing Microorganisms Produce So That the Plant Can Absorb Them?

Several processes, primarily dissolution–precipitation, and sorption–desorption, influence the phosphorus concentration in the soil. The primary mechanism by which soil microorganisms solubilize phosphorus involves the release of complex compounds or mineral solvents. These include organic acid anions, siderophores, protons, hydroxyl ions, and carbon dioxide. Additionally, various extracellular enzymes are released, further facilitating the solubilization process. This intricate biochemical interaction plays a crucial role in the phosphorus cycle within the soil ecosystem [ 125 , 126 ].

Organic Phosphate Mineralization

Plants cannot directly take up high-molecular-weight organic forms of phosphorus, which are generally transformed into inorganic forms after mineralization. Organic transformation and mineralization are catalyzed by different soil enzymes, mainly phosphatases that hydrolyze organic phosphorus into its inorganic forms, such as HPO 4 and H 2 PO 4 , so plant roots can absorb it through the soil solution. Phosphatases can hydrolyze both phosphate ester and anhydride bonds, including acid and alkaline phosphatases, diadenosine triphosphatases, exonucleases, phosphoprotein phosphatases, phosphodiesterases, 5′-Nucleotidase, phytases, and acid phosphomonoesterase. The mineralization of organic phosphorus is carried out by microorganisms in the soil and influenced by environmental factors and exceptionally moderate alkalinity. In soil, organic phosphorus is found in the form of inositol phosphate, phosphodiester, phospholipids, nucleic acids, pesticides, antibiotics, detergent additives, and other forms resistant to chemical hydrolysis by soluble forms [ 127 ]. Phosphatases are produced by many soils’ microbial species, including Aspergillus , Bacillus , Mucor , Penicillium , Rhizopus , Pseudomonas , and mycorrhizal hyphae . Acid phosphatases predominate in acid soils, while alkaline phosphatases predominate in neutral and alkaline soils. Plants and phosphatases release orthophosphate ions from organic P forms, making microbial phosphatases more efficient [ 126 , 128 ].

Regarding the mineralization of inorganic phosphorus, one of the most prominent mechanisms is the production of organic acids by microorganisms in the soil. These acids lower the pH of the soil rhizosphere or complex the cations responsible for the precipitation of phosphorus. These acids can compete with phosphorus for sorption sites in the soil or form soluble complexes with metal ions associated with insoluble phosphorus compounds, such as AlPO 4 , Ca 3 (PO 4 ) 2 , and FePO 4 , to dissolve mineral phosphorus [ 33 ].

Genetic Mechanisms of Phosphorus Solubilization

Some studies suggest that organic matter may function as the main reservoir (30–80%) of immobilized phosphorus, mainly in forms like orthophosphoric acid, phytic, inositol phosphates, phospholipids, and nucleic acids, which are degraded by the activity of enzymes, such as acid phosphatase, phytase, and nucleases, allowing plants or crops to absorb this element [ 33 ].

Organic Phosphorus Solubilization by Phosphatases

Organic phosphorus solubilization in soil occurs through the action of the enzyme phosphatase, which degrades macromolecules to micromolecules and converts them into water-soluble sources of phosphorus. These sources are transported through the soil pore network and recognized by the root systems of plants, biota, and microbiota [ 129 ]. P can be released from organic compounds by the enzymatic action of three groups of enzymes. The first group, the nonspecific phosphatases or phosphomonoesterases, performs dephosphorylation processes through a biochemical reaction that extracts phosphorus from phosphodiester or phosphoanhydro bonds in organic matter. The second group of enzymes, phytases, releases phosphorus from phytic acid. The third group comprises the enzymes phosphatase and C-P lyase, which break the C–phosphorus bonds in organic phosphonates [ 33 , 115 ].

Phytase enzymes are a type of phosphatase produced by microorganisms, such as Bacillus sp., Enterobacterium Burkholderia , and Azo spirillum , among others [ 130 ]. They allow for the release of insoluble phosphorus from the hydrolysis of phytates present in the phytic acid ( inositol hexaphosphate ) available in the soil. Phytic acid also acts as a storage source of Mg +2 and K + , ions, and, to a lesser extent, Ca +2 , Mn +2 , Ba +2 , and Fe +2 .

The solubilization of organic phosphorus is a process catalyzed by enzymes. Among these enzymes, the most studied are phosphatases, which participate in the dephosphorylation of phosphodiester groups attached to organic matter, and phytases, which catalyze the hydrolysis of phytic acid, sequentially releasing up to six free orthophosphate groups. Phytase enzymes, or Myo-inositol hexaphosphate hydrolases, are of great scientific and commercial interest due to their wide natural distribution in plants, microorganisms, and some animal tissues [ 115 ].

The activity of phytase enzymes is higher in microorganisms, such as Gram-positive bacteria ( Bacillus ), Gram-negative bacteria (such as Klebsiella [ 131 ] and Pseudomonas sp. [ 132 ]), filamentous fungi (such as mycorrhizal fungi and Aspergillus niger ), and other phylogenetic groups of bacteria [ 133 ]. Mainly, these enzymes are produced by Gram-negative bacteria in the intracellular zone of the periplasm, whereas in Gram-positive microorganisms, they are extracellular. Acid phytases with a pH between 2.5 and 5.5, belonging to the acid histidine phosphatases family, have also been identified. For soluble phosphorus to be produced by bacteria colonizing plant tissues, especially the root surface and interior, these microorganisms must become established in these tissues [ 134 ].

In Figure 15 , derived from Israel da Silva’s study [ 135 ], the mechanisms of phosphate solubilization are illustrated, demonstrating the phosphorus cycle and its mobilization. The base of the arrows contains numbers and symbols, each corresponding to a specific process involved in phosphorus mobilization, as detailed in the figure’s heading. These processes include Non-Specific Acid Phosphatases (NSAPs, 4.1.1), phytases (4.1.2), phosphatases (4.1.3), carbon–phosphorus lyases (C-P Lyases, 4.1.4), organic acids (4.2.1), inorganic acids (4.2.2), enzymes or enzymolysis (4.2.3), siderophores (4.2.4), exopolysaccharides (4.2.5), proton release (4.2.6), hydrogen sulfide (H2S) production (4.2.7), and direct oxidation (4.2.8). Each number corresponds to the section where the respective mechanism is explained in detail.

Phosphorus cycle and nutrient mobilization.

3.2.5. What Are the Colonization Processes in Rhizosphere Phosphorus-Solubilizing Microorganisms?

Plant–microorganism interactions are highly complex and dynamic. They start with the microbial colonization stage, which can occur on different plant surfaces, such as the rhizosphere and endophyte [ 136 ].

Rhizosphere Colonization

The rhizosphere is the soil area surrounding plant roots, and it is highly influenced by them. This environment is dynamic and has a great diversity of microorganisms [ 137 ]. Rhizospheric colonization begins with the migration of bacterial populations into the roots in response to the release of organic compounds from plants into the soil, known as photosynthates or exudates, which contain essential nutrients, such as amino acids, nucleotides, fatty acids, organics, phenolics, sugars, and vitamins, that are used by microorganisms as a source of nutrients for their growth [ 138 ]. The “rhizosphere effect” involves the capacity of plant exudates, which act as chemoattractants of beneficial microbial communities and thus increase microbial densities in concentrations in the order of 108 to 1012 UFC/g soil, while bacterial concentrations in soil regions that are far from the root system of plants present concentrations of less than 106 UFC/g soil [ 139 ].

Once rhizospheric microorganisms establish contact with the root surface, they can form microcolonies and, depending on the type of microorganism, remain on the surface or progress to endophytic colonization [ 139 ]. Bacteria have been reported to respond to plant exudates by expressing various genes, such as those associated with synthesizing exopolysaccharides for biofilm formation [ 140 ], which protect microbial communities from adverse environmental factors [ 141 ].

Bacillus subtilis is attracted to L-malic acid secreted by Arabidopsis thaliana . This compound activates biofilm formation in a process that depends on the same genes required for biofilm formation in vitro [ 141 ]. In the case of Azospirillum brasilense , it is a PSB-type soil bacterium that promotes the growth of grasses, such as wheat and corn, colonizes the rhizoplane, and forms biofilms that allow it to compete for space with indigenous soil microorganisms [ 142 ]. On the other hand, the formation of biofilms with Paenibacillus polymyxa in the roots of Arabidopsis thaliana and barley has been reported, demonstrating the colonization pattern by using a strain labeled with GF-phosphorus protein and microscopic techniques. There are several pieces of evidence suggesting the crucial role that exopolysaccharide production plays in biofilm formation in the early stages of plant–microorganism interaction and allows it to compete with other microorganisms that are colonizing the rhizosphere and rhizoplane zone of the root [ 143 ]. This interaction is essential in determining the effects of plant growth promotion; without it, growth-promoting soil bacteria cannot improve plant nutrition or stimulate growth by producing phytohormones. Therefore, on many occasions, biofertilizers do not have positive effects due to the lack of plant–microorganism interaction [ 144 ].

Endophytic Colonization

Endophytic colonization refers to the colonization of internal plant tissues by bacteria called endophytes. These microorganisms can be isolated from plant tissues with surface disinfection, do not cause visible damage to the plant, and can promote growth [ 145 ]. Conceptually, endophytic bacteria are defined as those bacteria that can be isolated from plant tissues with surface disinfection and do not cause visible damage to the plant. However, this definition does not include non-culturable endophytic bacteria because of experimental limitations [ 146 ]. Most endophytic microorganisms come from or are recruited from the rhizosphere environment community. Therefore, they must be efficient colonizers of the rhizosphere and rhizoplane [ 147 ]. Successful endophytic colonization implies compatibility between the host and the endophyte. Furthermore, it is suggested that endophytes are specialized members of the rhizoplane colonizing community and that the plant selects the bacteria most adapted to endophytic life from a large pool of rhizospheric bacteria. Although it is unknown whether endophytic bacteria need to colonize a specific tissue or organ to perform their functions adequately, it is postulated that stochastic, environmental events and bacterial-dependent factors influence their ability to colonize internal plant tissues.

Concerning spatial distribution within the plant, although a more significant presence of endophytic bacteria is generally observed in roots, their colonization has been reported in other organs, such as stems, seeds, leaves, fruits, tubers, reproductive organs, and nodules [ 148 ]. Endophytic bacteria from the rhizoplane to the root cortical tissue can enter through passive and active mechanisms and various routes of entry [ 146 ]. Passive entry can occur through natural cracks in the lateral root emergence zones (via the middle lamina of the epidermis), through the base of root hairs and in the root apex growth zone, or by those caused by other microorganisms [ 149 ]. Several studies have reported the colonization of endophytic bacteria in the root through root hairs. Other sites commonly used by endophytes to colonize plants are the stomata, lenticels, and radicles [ 150 ]. Various entry mechanisms exist for microorganisms to penetrate plant tissues, with one involving active processes that rely on lytic enzymes capable of breaking down the plant cell wall. Among these enzymes are pectinases and cellulases, which play a crucial role in creating fissures or cracks in the rhizodermis, thus providing a pathway for the microorganisms to enter. The enzymatic activity appears to be facilitated by passing through the endodermis, which enables penetration into the pericycle and xylem vessels [ 150 , 151 ]. Notably, the ability to degrade pectins found in the primary plant cell wall and lamina media is a shared characteristic among many plant-associated bacteria, including endophytic ones.

Endophytic bacteria employ active mechanisms to penetrate plant tissues, which involve the production of lytic enzymes that degrade the plant cell wall. These enzymes, such as pectinases and cellulases, favor the formation of fissures or cracks in the rhizodermis, through which the microorganisms enter. The enzymatic activity seems to be mediated by passage through the endodermis, which allows for penetration into the pericycle and xylem vessels [ 150 , 151 ]. The ability to degrade pectins of the primary plant cell wall and lamina media is a common feature of many plant-associated bacteria, including endophytic bacteria.

The primary distinction between pathogenic and endophytic bacteria resides in the type of lesions they induce in the plant tissues they inhabit. Additionally, the ability to secrete cellulolytic enzymes plays a crucial role in enabling the breakdown of the plant cell wall, thereby facilitating vertical dissemination. For instance, Azoarcus sp. strain BH72 does not rely on cellulases for its growth, as it cannot utilize Carboxymethyl cellulose as the sole source of carbon and energy. Consequently, its function may lie in promoting the invasion of tissues during root colonization.

Unlike phytopathogenic organisms, which cause aggressive damage to plants, it has been shown that Azoarcus sp. does not secrete its cellulolytic enzymes but that these are associated with its cell surface. Thus, it is postulated that they may mediate more localized and less damaging plant cell wall digestion. Hurek et al. [ 152 ] reported that Azoarcus sp. can enter the intercellular spaces of the grassroots and increase cell density, mobilizing between the central vascular tissues of plants until reaching the aerial tissues, such as stems and leaves, where it generates growth promotion and phosphate solubilization. These findings have been reported in several studies [ 93 , 115 , 132 ].

3.2.6. What Are the Prospects for Phosphorus-Solubilizing Microorganisms as a Biotechnological Solution?

Integrating phosphorus-solubilizing microorganisms into sustainable agricultural practices begins with identifying and selecting strains that exhibit high phosphorus solubilization efficiency [ 153 ]. This requires a literature review to determine strains previously reported for their phosphorus-solubilizing ability. In addition, it is imperative to isolate and characterize new strains from diverse sources, such as soils, rhizospheres, endophytes, or phyllospheres of crops or other plant species [ 154 ]. This process involves the evaluation of phosphorus solubilization capacity under controlled laboratory conditions, which facilitates the selection of strains that demonstrate superior solubilization potential [ 135 ].

Following laboratory selection of phosphorus-solubilizing strains, culture conditions should be optimized to increase their proliferation and efficiency, including determining the optimal pH, temperature, and nutrient conditions that promote microbial growth and translating these conditions from laboratory to large-scale culture [ 155 ]. Secondary metabolites produced by solubilizing microorganisms, which play a role in phosphorus solubilization, should also be evaluated. Once optimal growth conditions for the microorganisms have been achieved, controlled trials with various agronomic interest crops and different soil types will be conducted [ 156 ]. These experiments should be designed to evaluate the appropriate dosage for each crop, evaluate soil persistence, and determine the solubilization efficiency in different soil types. Applying selected microorganisms will allow for the measurement of phosphorus availability in the soil and its impact on crop growth and yield, providing crucial data to fine-tune application methods prior to field trials [ 157 ].

A critical step towards the large-scale application of this biotechnology is the development of field trials to validate the biotechnology and evaluate the efficacy of phosphorus-solubilizing microorganisms under natural conditions. Experimental plots of each soil type, climatic condition, and dose are needed. Treatments with phosphorus-solubilizing microorganisms should be applied together with traditional agricultural practices to evaluate their impact on soil phosphorus availability, plant growth, and crop yield [ 158 ]. This step is vital to confirm the practical feasibility and the economic and environmental benefits of this biotechnology, as the high solubilization efficiency often observed in the laboratory may not translate into significant effects on crops under natural conditions [ 159 ].

After field confirmation of the efficacy of the solubilizing microorganisms, the next step is developing the commercial product. This process involves investigating different formulations (liquid, solid, encapsulated) that maximize the viability and efficacy of these microorganisms [ 160 ]. The formulations developed must also undergo stability and shelf-life testing to ensure their quality over time and to obtain the necessary regulatory approvals by national and international standards, which are essential for commercializing these products domestically and internationally [ 161 ].

The successful adoption of phosphorus-solubilizing microorganism biotechnology in agriculture may depend on effective training and technology transfer to small, medium, and industrial farmers [ 17 ]. To adopt this biotechnology, it is crucial to develop training programs and educational materials that elucidate the use and benefits of the product, both from an economic and environmental point of view. In this regard, workshops, seminars, and field demonstrations should be organized to familiarize farmers with these new technologies and establish collaborative networks with agricultural cooperatives and rural development organizations, which is also essential to facilitate adoption [ 161 ].

Finally, monitoring systems must be established to assess the long-term impact of the application of this biotechnology in agriculture. This includes monitoring the availability of phosphorus in the soil and crop yields and reducing the use of chemical fertilizers. These data will allow for the assessment of economic and environmental impact and facilitate the publication of reports and case studies highlighting successful experiences and lessons learned with the developed biotechnology product.

4. Threats to Validity

This systematic literature review on phosphorus-solubilizing microorganisms as a biotechnological alternative identified and addressed several potential threats to validity. These threats are categorized into four main types: internal validity, external validity, construct validity, and conclusion validity.

• Selection bias

The investigators’ subjective judgments may have influenced the selection of primary studies, leading to the inclusion or exclusion of relevant studies. To mitigate this, rigorous and predefined inclusion and exclusion criteria were employed, and several investigators independently reviewed studies to reduce individual bias.

• Language bias

The review focused primarily on English-language publications, which could overlook valuable research published in other languages. Although we acknowledge this limitation, we included studies with abstracts in English regardless of the language of the full text to mitigate this.

• Temporal validity

The rapid advancement of biotechnology may make some older studies less relevant to current practices. To mitigate this, we analyzed the publication dates of included studies and gave more weight to recent research while still recognizing foundational work.

• Researcher expectations

Researcher expectations could inadvertently influence the interpretation of results. To mitigate this, we employed multiple reviewers for data extraction and interpretation and used standardized forms to ensure consistency.

By recognizing and addressing these threats to validity, we aim to improve the robustness and reliability of this systematic review. We recognize that eliminating all threats is impossible, but by transparently reporting these potential limitations, we provide readers with the context necessary to interpret our findings critically. Future research should address these limitations to advance our understanding of phosphorus-solubilizing microorganisms as a biotechnological alternative.

5. Conclusions and Future Work

This study involved conducting a bibliometric analysis and literature review to examine the status of the domain of phosphorus-solubilizing microorganisms and their role in improving phosphorus availability for plant uptake. Different bibliographic databases were used to collect the different scientific papers, totaling 2322 research papers. Subsequently, the methodology proposed by Donthu and Jia’s guide was used to ensure a systematic and rigorous approach to the literature review. The bibliometric tools, such as R-studio and Bibliometrix, facilitated the visualization and analysis of the data. The results indicate a growing interest in research on these microorganisms since the 2000s, highlighting their importance in the context of sustainable agriculture and phosphorus scarcity. Notably, India leads the research output with 542 published papers, followed by China with 336, Brazil with 225, Pakistan with 217, and the United States with 158 publications. Together, these countries account for 52.6% of the total publications, indicating their significant contribution to the domain; this may be due to their extensive agricultural sectors and demand for phosphorus fertilizers. The analysis also showed relatively limited international collaboration, suggesting the possibility of further collaboration between countries.

The study also provided information on phosphorus solubilization mechanisms and the main microorganisms involved. Phosphorus is an essential macronutrient for plants, but it has reduced availability; this situation can be overcome by releasing organic acids, phosphatases, phytases, and other compounds that liberate phosphorus from insoluble compounds. Several genera of bacteria can solubilize phosphorus, especially Bacillus , Pseudomonas , Burkholderia , and Rhizobium . These microbes undergo complex rhizospheric and endophytic colonization processes to interact with plant roots and deliver phosphorus. However, to achieve phosphorus solubilization and ensure that it is efficiently provided to plant species, it is required that PSB microorganisms develop an effective interaction with plants to favor the growth of this type of microorganism. For this, the plants, and especially the root, must provide secondary metabolites. If this interaction fails to be effective, as it has been evaluated for several plant-growth-promoting inoculants, plants cannot obtain soluble phosphorus, decreasing their nutrition.

In addition, it was found that the use of alternative methodologies to find solutions to phosphorus deficits in soils is promising due to the high diversity of phosphorus-solubilizing microorganisms with the biotechnological potential to transform insoluble phosphorus, which is reserved in soils as soluble phosphorus, contributing to the improvement of plant growth and development. In this way, farmers can lower the production costs of their crops, in addition to having cleaner production, as they avoid the excessive use of chemical fertilizers, which contain heavy metals, strong acids, such as nitric acid, beneficial acids in the soil, contaminate aquatic ecosystems, increase their acidity, and ultimately interfere with plant growth, thus affecting environmental dynamics.

This research highlights phosphorus-solubilizing microorganisms’ potential as a sustainable alternative to conventional phosphorus fertilizers. However, further research is needed on these microbes’ diversity, efficacy, and applicability in different environments and crops. Further international collaboration and research on microbial mechanisms can pave the way for effectively integrating this technology into agricultural systems. The results of this study provide a valuable basis for guiding future research in this emerging field.

Finally, regarding future work, one could talk about exploring the diversity of efficient phosphorus-solubilizing microorganisms and their bioprospecting in diverse environments, elucidating the molecular pathways involved in the production of organic acid, phosphatase, and phytase, and the study of synergistic effects between solubilizing microorganisms and mycorrhizae.

Acknowledgments

Thanks to the Corporación Universitaria Comfacauca—Unicomfacauca for providing its facilities and for their help in conducting the research.

Funding Statement

This research did not receive external funding, and the APC was funded by the Universidad Nacional Abierta y a Distancia-UNAD.

Author Contributions

Conceptualization, A.F.S.P., Z.Y.D.E., and E.V.R.C.; methodology, A.F.S.P., Z.Y.D.E., and E.V.R.C.; software, A.F.S.P., Z.Y.D.E., and E.V.R.C.; validation, A.F.S.P., Z.Y.D.E., and E.V.R.C.; formal analysis, A.F.S.P., Z.Y.D.E., and E.V.R.C.; writing—review and editing, A.F.S.P., Z.Y.D.E., and E.V.R.C. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Conflicts of interest.

The authors declare no conflicts of interest.

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The “breakdown” in the subtitle of Emily Witt’s haunting new book, “Health and Safety,” isn’t hers. It belongs to Andrew, her boyfriend of four years; he started behaving erratically when pandemic lockdowns in 2020 put an end to the underground party scene in Brooklyn’s Bushwick neighborhood, where Witt and Andrew had danced, done drugs and found friends.

But Witt also writes about a bigger breakdown, one that eroded the boundaries between their subculture and the world at large. “It was like we were a part of a big, safe bubble,” Witt recalls of the time before the pandemic. While American politics were getting meaner, she and her friends believed themselves “to be articulating a kind of new moral order,” full of care and concern for one another. It’s a lovely, earnest sentiment. But Witt, the author of “ Future Sex” (2016) , writes with such cool precision that it’s hard to imagine her fully losing herself in sentimental projects, even with chemical assistance. And a bubble, of course, is never safe for long.

“Health and Safety” braids together several narrative strands: the parties, the music and the drugs; Witt’s relationship with Andrew; and her work as a journalist during the Trump years, much of it for The New Yorker . Until she turned 31, Witt’s pills were sanctioned by prescription; she took Wellbutrin, which she liked because it treated her depression while keeping her skinny and productive.

When she stopped taking Wellbutrin, she embarked on a “course of study,” deciding “to try as many psychedelic drugs as possible.” In the spring of 2016, she met Andrew. They went to raves together, took drugs together and eventually moved in together. She was 35; Andrew was five years younger. They chose an open relationship. “A wedding now seemed like a dumb pageant,” she writes, “a false promise” promoting “a patriarchal model of family” that “was the lie of fascism.”

There’s a part of Witt that genuinely believes her own contempt. But another part of her recognizes that righteous sloganeering can’t fully account for the various gradations of what she feels. One of the things she loved about the music and the drugs is the way they opened up another space beyond language. The electronic sounds of techno “did not say what to feel or when to feel it,” she writes. “It had the discontinuity of poetry instead of the continuity of a story or a novel; it pursued a different order of sense-making.”

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