Review Reports - A Bibliometric Anatomy of Literature on Bio-Based Fertilisers with Insights into Environmental Impacts and Evaluation Approaches

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Referee’s report: environments-3831884

Bio-Based Fertilisers and the Environment: A Data-Driven

Review of Trade-offs, Research Priorities and Sustainability Gaps

This is a valuable and timely review of recent trends in a particular sector of non-renewable fertilizer research. The review demonstrates that this is still a developing field; an aspect that came as a surprise to me, and probably many other readers. I recommend publication with some minor amendments. The authors might wish to consider the points made below.

The authors set 2001 as the starting point of the publications reviewed, which is a reasonable strategy. However, maybe a mention should be made about non-renewable fertilizer research before 2001 and that recent bio-based fertilizer (BBF) research builds upon earlier foundations. For millennia livestock manures were the principal means of maintaining soil fertility. Manures and slurries are still extensively used today, and much research has been devoted to their efficacy.

From 1850 until about 1930, sodium nitrate guano, mined as “Chilean nitrate” was a widely used BBF until it was depleted and displaced by the Haber-Bosch process. The organic agriculture movement has for decades also conducted extensive research on the utilization of aerobic compost, vermicompost, mulching with crop residues and processing by-products, and applications of livestock manures.

It is clear that the authors have some specific ideas in mind when they consider BBFs. They should therefore define what they mean by BBFs and how the use of these follow on from previous practices to conserve and recycle nutrients. For example, in lines 75 and 76, the authors write, “Farmers in several European countries, such as Belgium, Denmark, France, and Germany, have shown a growing acceptance of BBFs”. I would argue that they have been using BBFs for many centuries. Thus, a clear definition of what the authors mean by BBFs and their grounds for excluding extensively used traditional practices from their review, are needed.

The distinction between evaluation of BBFs by empirical research and Life Cycle Assessment (LCA) is novel and valuable. LCA has a place in evaluation, but it is subject to the accusation of becoming the new “dismal science” (Thomas Carlyle, 1849). By its nature LCA identifies problems and disadvantages (q.v. lines 434 – 442). Rigorous comparisons with the non-renewable alternatives might better highlight advantages. It may be that BBFs are so reliant upon energy and resource intensive inputs to achieve nutrient recovery or conversion from organic to ionic nutrient forms, that their use may create more problems than they solve. It needs to be borne in mind that LCAs are models and are therefore dependent upon assumptions and algorithms derived from empirical experimentation (q.v. lines 485-487) which may have varying degrees of applicability, depending on circumstances (q.v. lines 493-494).

Minor corrections required:

Lines 35 and 301: Both phosphorus and phosphorous variants are used, the latter as a search term. Use one form or the other consistently.

Line 43: Insert “[1]” instead of “Lal, 2002”.

Line 210: Delete “2015”

Line 333: Remove upper case from “Phosphorus”

Line 368: “America” is too imprecise. It should be “North America” or “USA”.

Author Response

Thank you for dedicating your time to reviewing this manuscript. Please find our detailed responses below, and the changes are highlighted in yellow in the new version of the manuscript:

Reviewer’s suggestions:

We fully agree with the reviewer’s observation that many of these products have been used by humans for centuries. To clarify our use of the term “BBFs” and avoid confusion with traditional practices, we have added a footnote in line 74. This footnote provides a definition of BBFs provided by some of the co-authors in Egas et al 2023 that is consistent with the European regulatory framework and the scope of the several European projects and the European Sustainable Phosphorus Platform (ESPP) DOI:10.5281/zenodo.13969019, which focuses on innovative bio-based fertilisers developed through modern processing technologies. This clarification aims to distinguish BBFs as considered in our study from historically used organic amendments.

In agreement with the reviewer’s comment, a substantial part of the manuscript discusses the strengths and limitations of both LCA and non-LCA methodologies. As part of our concluding remarks, we suggest that future research should aim to develop a more holistic assessment framework (L-497 to L-500).

Minor comments

Lines 35 and 301: Both phosphorus and phosphorous variants are used, the latter as a search term. Use one form or the other consistently.

Response: We replaced all the terms with “phosphorus” to bring consistency

Line 43: Insert “[1]” instead of “Lal, 2002”.

Response: The change was made

Line 210: Delete “2015”

Response: Deleted

Line 333: Remove upper case from “Phosphorus”

Response: The uppercase P was changed to lowercase

Line 368: “America” is too imprecise. It should be “North America” or “USA”.

Response: We replaced “America” with North America

Reviewer 2 Report

Comments and Suggestions for Authors

The paper by Torres-Guerrero et al reports a bibliometric analysis of research related to bio-based fertilizers (BBFs), and a description of the 10 most cited papers that used life cycle assessment (LCA) and non-LCA methods to evaluate the potential impacts of BBFs.

The paper is well researched an well-written in general. It describes the trends in publications on BBFs over time, their main contributors, journals, and research networks. The authors also describe the content of 20 of the most cited papers, mainly presented in two tables. Whereas the bibliometric analysis is well presented, the 'in depth' content review is disappointing.

The paper is poorly structured and, after a rather difficult read, the reader does not gain a good overall picture of research results (i.e., trends and results are different). In this respect, the title is somewhat misleading as the paper cannot be described as a data-driven review of trade-offs, or sustainability gaps; although it does address priorities to some extent.

A systematic review would demand that more in dept knowledge is presented on types of BBFs - perhaps broken down into categories, types of impact (including both LCA and non-LCA methods), current uses, policies, etc. The inclusion of two large tables that summarize 20 papers is not sufficient as a review of the topic and is not systematic.

I would suggest that the authors include more of an overview of the topic in the introduction, including a table of definitions and types of BBF; then present their bibliometric analysis and call for greater collaboration or make other recommendations based on the analysis. The authors should revise their claims in lines 128-130, particularly the suggested second and third contributions to gained knowledge.

The 'content analysis' should be left out because it is not thorough enough (less than 10% of papers were read for content). A proper review would take all 247 papers and systematically explain what they contain - including in the references all 247 papers. In any case, tables 3 and 4 are too cumbersome and not sufficiently summarized to appear in an international publication.

I include some minor points/suggestions below and wish you the best with your revisions:

Multiple references should be within a single set of brackets – see lines 42, 56, 77 for example

Lal 2020 reference not numbered – line 43

54 more sustainable agricultural systems

56 suggest ‘nutrient management protocols’ instead of fertilization

60-66, 84-95 is under referenced

120 there = these

139-141 This does not seem to be part of a structured review, if the terms were just for use in meetings, etc.

151 remove ‘author’

154 review use of the word filtered – perhaps maintained

204-205 no need to explain why trends occurred – this could be due to more journals published, and a global increase in articles in English for example

208 associated – more likely coincides; i.e, do not speculate

222-230 no need for uppercases with subjects (e.g., energy not Energy)

In figure 5, what is the difference between lca and life cycle assessment, and wastewater and wastewater treatment

330-336 revise appearance of numbered references here – it is difficult to understand the sentences

Figure 7 include colour code with the figure or legend

Figure 8 what do the colours in part b indicate – for example, Spanish universities appear in green, blue and brown

Author Response

Thank you for dedicating your time to reviewing this manuscript. Please find our detailed responses below, and the changes are highlighted in yellow in the new version:

2. Reviewer’s suggestions:

Response We sincerely thank for your thoughtful and detailed feedback. Both their input and that of the other reviewers have been extremely helpful in improving the manuscript. In response, we have revised the title and clarified the scope and limitations of the study to better reflect its objectives and methodological approach.

Regarding the in-depth analysis, the selection of 20 articles was based on a structured methodology aimed at identifying key trends and knowledge gaps. This process involved manual filtering and prioritization, which is now explicitly described in the methodology section. While we acknowledge the limitations of this approach, we believe it offers valuable insights into the most influential studies and the diversity of assessment methods used.

As for Tables 3 and 4, although they are dense, we consider them essential for providing a concise synthesis of the most cited studies, including the methodologies applied, environmental concerns addressed, and the life cycle stages evaluated. These tables support the identification of methodological variability and the need for harmonization, which are central findings of the paper.

We appreciate the reviewer’s suggestions regarding the structure and content of the manuscript and will continue refining the presentation to enhance clarity and coherence.

2. Minor points:

Lal 2020 reference not numbered – line 43

Response: The change was made

L-54 more sustainable agricultural systems

Response: The change was made

56 suggest ‘nutrient management protocols’ instead of fertilization

Response: We replace monitoring and fertilisation protocols with “nutrient management protocols”

60-66, 84-95 is under referenced

Response: The references were added

120 there = these

Response: The change was made

139-141 This does not seem to be part of a structured review, if the terms were just for use in meetings, etc.

Response: The wording of the sentence was improved to prevent the meaning of the sentence from being interpreted as meaning that the terminology search was only for casual use.

151 remove ‘author’

Response: The change was made

154 review use of the word filtered – perhaps maintained

Response: The word “filtered” was replaced with “maintained”

204-205 no need to explain why trends occurred – this could be due to more journals published, and a global increase in articles in English for example

Response: The explanation was removed

208 associated – more likely coincides; i.e, do not speculate

Response: The change was made, replacing “can be associated” with “coincides”, avoiding the speculation.

222-230 no need for uppercases with subjects (e.g., energy not Energy)

Response: The changes were made

In figure 5, what is the difference between lca and life cycle assessment, and wastewater and wastewater treatment

Response: In Bibliometrix, the three-field plot visualization is generated directly from the metadata fields (e.g., authors, keywords, sources) as they appear in the original dataset. This means that synonymous terms—such as "LCA" and "Life Cycle Assessment"—are treated as distinct entries unless they are manually standardized before the analysis.

The tool does not currently support dynamic grouping or merging of synonyms within the plot itself. To address this, a manual preprocessing step is required, such as creating a controlled vocabulary or thesaurus to unify terms prior to generating the plot. However, this process can be complex and may introduce subjectivity, especially in large datasets.

In this study, we chose to preserve the original descriptors to maintain transparency and reproducibility of the bibliometric analysis. Nonetheless, we acknowledge the presence of synonymous terms and have discussed their equivalence in the methodology and interpretation of results.

330-336 revise appearance of numbered references here – it is difficult to understand the sentences

Response: The sentence was rewritten by adding the names of the authors to enhance the reading

Figure 7 include colour code with the figure or legend

Response: The suggestion was rejected, a colour legend was not included because bibliometric visualisations follow well-established conventions: node size denotes citation frequency, edges represent co-citation strength, and colours indicate clusters of related publications. Consequently, the majority of published studies employing this type of analysis do not provide a legend within the figure. Nevertheless, to enhance clarity, we have revised the figure caption to explicitly describe the meaning of nodes, edges, and colours.

Figure 8 what do the colours in part b indicate – for example, Spanish universities appear in green, blue and brown:

Response: Idem that the previous and the figure caption were improved.

Reviewer 3 Report

Comments and Suggestions for Authors

While this manuscript is well structured and mostly well written, I found it to be disappointingly shallow and not delivering what it said on the tin (i.e. the title). It might be better titled, “A bibliometric anatomy of literature on bio-based fertilisers.” It really does not go beyond the bibliometrics. It does not discuss what the environmental impacts are, in comparison with chemical fertilisers that BBFs might displace, or where the technological and analytical gaps currently lie. It contains some well-presented figures, but they convey little important information about the field of bio-based fertilisers. I got the feeling that the authors are trained as librarians or social scientists and lack both interest in and understanding of the topic they are handling.

The abstract claims, “This study provides a comprehensive evaluation of BBFs” where, at best, it provides a deep exploration of what the latest bibliometric apps can tell us about a limited sample of the literature. In no measure does it touch on “evaluation of BBFs”. We are treated with lists of the most used journals and the most cited authors and their collaboration networks. The important observation was made that the European Commission’s policy on the “circular economy” has apparently elevating funding and activity in the bio-fertiliser field. We learn nothing about the current state of technological, economic or environmental challenges to nutrient recovery or market penetration in terms of displacing chemical fertilisers.

The authors’ main conclusion is that the field could do with more consistent methodologies, presumably to make bibliometric analyses easier. They don’t consider that a diversity of methods might be appropriate for such a wide diversity of potential research questions and contexts.

I do not think this study adds sufficient value to the literature to warrant publishing. If it were published, it should be under a different title, such as the one I have suggested. This sort of study should act as groundwork for a researcher's own entry into the field, but does little to help any other researcher hoping to do so. The summaries of a few well-cited papers in Tables 3 and 4 might prove helpful, but in a limited way.

Following are specific editorial notes:

Abstract:

It is surprising that you assess the Life cycle analysis (LCA) papers as less data-driven than non-LCA. Perhaps the distinction should be between empirical field work generating primary data and meta-analyses using secondary data?

L 37: “N-based fertilisers require significant fossil fuel use for the Haber-Bosch process” – technically, they don’t “require” fossil fuels (the N itself comes from the atmosphere), but to date natural gas has been the main feedstock for both energy and hydrogen.

L 60-62: You don’t mention sewage (or, as you later term it, municipal wastewater), the single biggest channel for intercepting nutrients.

L 74: “reducing the GHG emissions” could do with elaboration. Are you talking specifically about N₂O emissions or life-cycle energy use?

L 98-101: It seems odd to mention “ecotoxocological methods” without mentioning chemical analysis to identify toxic components.

L 107: “from an environmentally sustainable perspective.” I think you mean “from the perspective of environmental sustainability.”

L 113: “necessary” should read “necessity”.

L 134: So it turns out you only read through 20 articles – hardly enough for a meaningful literature review.

L 151: “criteria selection” should be “selection criteria”; “was” should be “were”.

L 153: by “original”, do you mean “peer reviewed”? (In contrast to “grey literature”)

Section 2.2: It saddens me to see normal processes of literature review (gathering, reading and critically evaluating relevant papers) presented as cutting-edge methodological process, distracting from the fact that only 20 papers were actually read for this review! Other reviews were excluded from the 20 selected. While certain conclusions can be drawn from search-terms and abstract reviews, the evolution of cutting-edge thinking cannot. It makes for a light-weight effort.

For instance, citations of papers can be in support of the cited paper or critical of it.

L 201: “The most significant numbers” meaning what? The numbers quoted are merely the sample of papers caught in this particular Scopus search. They say nothing about whether “research on BBFs has increased in recent years.”

L 210: Delete “2015”.

L 270-271: Two journal titles are included in the single set of quotation marks, thus appearing to be one title, making “is ranked 11th and 14th, respectively” very confusing. Add quotation marks to separate these two, and change “is” to “are”.

L 276: “1,101 authors have contributed” again presents your sample of papers as being a complete collection.

Figure 6a: I can’t find any explanation of what dimensions 1 and 2 represent.

Figure 6b: presumably the dots are placed on the first, last and average (or is it median?) year of publication but this is not explained. There doesn’t seem to be any logic in the order in which the key words are listed.

L 333: “iii) [45] research focused on Phosphorus recovery from municipal wastewater using an integrated comparative technological, environmental and economic assessment of P recovery technologies” seems to be an inappropriately long descriptor, perhaps pasted from the paper in question, and confuses the remainder of the sentence. Perhaps you could say, “[45] comparing the environmental and economic outcomes of different technologies for phosphorus recovery from wastewater, show …”

Figure 7 would be improved with an explanation of the differing size of text, and perhaps listing the themes of the clusters by colour in a legend rather than depending on the preceding text to explain them.

L 449: “Most studies favour the attributional LCA (ALCA) approach, with the consequential LCA (CLCA) being rarely used, in the list of the 10 most only [65] used this approach.” You could include a brief explanation of what “attributional” and “consequential” mean in this context. The final clause in this sentence does not make sense and needs restructuring.

Author Response

Dear Reviewer,

Please see the answer in the document attached

We appreciate all your inputs!

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The papper aims to systematically evaluate the environmental concerns, trade-offs, and sustainability gaps surrounding bio-based fertilisers (BBFs). The authors employ a hybrid methodology combining bibliometric and content analysis, covering publications from 2001 to 2024, to map key trends, environmental impacts, and assessment approaches in the field. The scope sits comfortably within the thematic framework of Environments by MDPI, as it addresses sustainability, life cycle assessment, and circular economy within agricultural systems, which are core concerns of the journal.

The paper’s structure is coherent and comprehensive, reflecting a clear narrative flow from context and methods through to findings and implications.

Some points, however, would benefit from improvement as following suggested.

Firstly, while the bibliometric data is exhaustively described, the resulting figures and workflow diagrams referenced in the text would need to be present and suitably captioned for the reader to benefit fully from the data visualization. There are several places where placeholders for figures remain (e.g., “Figure 1”) or partial figure legends appear, but the actual visuals are absent; this needs to be addressed for clarity.

The paper sometimes moves between broad summaries and detailed technical commentary without smooth transitions. The tables reporting top-cited papers are dense and could be improved by clearer formatting or more concise synthesis in the main text to guide readers through key findings and their implications.

The discussion on methodological gaps, especially the lack of standardized assessment frameworks for BBFs, is well-argued but could go further in recommending specific pathways towards harmonization. While the manuscript concludes with a strong emphasis on the need for integrated frameworks that combine LCA with direct environmental performance metrics, it could be improved by offering more actionable recommendations or highlighting recent relevant policy or methodological initiatives in this direction.

Several sections, particularly towards the end, are occasionally repetitive or restate points made earlier. A careful editorial review for conciseness and to avoid redundancy would strengthen the paper’s impact.

Overall, the paper largely achieves its aims by presenting a comprehensive literature synthesis of environmental issues in BBF research. However, before publication, minor revisions are recommended.

Author Response

Dear Reviewer,

Please see the answer in the document attached

We appreciate all your inputs!

Author Response File: Author Response.pdf

Reviewer 5 Report

Comments and Suggestions for Authors

The article submitted for review is interesting. It comprehensively presents the impact of biofertilizers on the natural environment. In my opinion, the concept of the article was well-thought-out, and the method of preparing a scientific (review) article is well-thought-out. Such a study is necessary in the literature on the subject. I realize that its content is unacceptable among opponents of such fertilizers. Comments on the article:

The purpose of such research should be further described, i.e., the aim and hypothesis should be reformulated.
It might be worthwhile to include another chapter presenting the most important results of the previous articles.

After these corrections, it will be suitable for publication.

Comments on the Quality of English Language

The purpose of such research should be further described, i.e., the aim and hypothesis should be reformulated.
It might be worthwhile to include another chapter presenting the most important results of the previous articles.

After these corrections, it will be suitable for publication.

Author Response

Dear Reviewer,

Please see the answer in the document attached

We appreciate all your inputs!

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript is significantly improved since the first submission. The new title adequately describes the study. While it provides little information about BBFs, it is a somewhat useful guide to the available literature and a nice methodological example of bibliometric analysis. I therefore recommend its publication.

Author Response

Dear reviewer,

Thank you so much for your kind, thoughtful, and detailed feedback. Your comments really helped us strengthen and refine the manuscript. We are very glad to hear that the revised version now better fits the study’s scope and that you find the methodological approach useful.

We truly appreciate your recommendation for publication and your valuable contribution to the review process.

Best regards,

Carlos T.