The Structural Quality of Soil Organic Matter under Selected Soil Fertility Management Practices in the Central Highlands of Kenya

Round 1

Reviewer 1 Report

Thank you for the opportunity to read this manuscript. It was inspiring to see an experimental setup with so many factors and enough repetition (4 is a lot more than 3!) to draw statistical conclusions. The study of management on nutrient availability and the stability / dynamics of soil organic matter is critical, and the authors applied the correct tools to do so.

 

However, the manuscript as written now needs considerable revision. There are substantial grammatical errors throughout that often limit interpretation. The introduction flows fairly well; important points are introduced to set up the problem, the more complicated method of NMR is well-introduced, and the experiment is adequately justified. 

 

The discussion is much too long, and no narrative arc is established at the beginning or emerges throughout. It feels that there are at least two “voices”/writing styles between section 4.1 and section 4.2. Section 4.2 is much easier to follow, but again is too long. Try to distill the main messages into take-away points that are supported by arguments from the literature. To me, it seems like the discussion paragraphs follow the structure of “Repeat a result, present an interpretation of the processes that give that result, and tie in a reference that does not obviously support that claim”. Rather than write two separate sentences for your result + attribution, try to blend them to achieve a more active voice. 

 

I would recommend re-writing the discussion to focus on a few main points that can be summed up at the end to draw conclusions about management. The experimental design and data are there, but the story is lost in the jumble of repeated results (which belong in the results section) and inconsistently connected interpretations. 

 

An important point about the scientific interpretation of these results is the concept of lignin being inherently resistant to decomposition in soil. The soil organic matter community has largely concluded that organic matter does not possess much inherent recalcitrance; rather, SOM is stabilized through its interactions with soil minerals and aggregates. Obviously, SOM decomposition is a continuum, and the authors may come to whatever conclusions they decide. But I would recommend starting with the paper by Lehmann and Kleber from Nature in 2015, DOI: 10.1038/nature16069

 

Finally, a good proof-read for English language, sentence fragments, repetition, and errant punctuation would improve the readability quite a bit. I know that citation formatting can be a nightmare and that this is not a final, but it’s something to consider for readability as well. 

 

I wish the authors luck in distilling the findings from this well-conceived study into a narrative that moves the science forward. The tone of this review was never intended to be harsh, but helpfully critical; this reviewer apologizes for any sharp language. I look forward to a revised version of this manuscript.

 

Below is an uncomprehensive set of line by line comments.

 

Abstract

 

Use of SOM and SOC interchangeably?

 

L21: “enhancing SOC stocks”

 

Include more of interpretation in your abstract that supports the final sentence, which can also be stated more assertively.

 

Fig.1: Maybe a line marking statistical significance would help improve readability and to identify the most significant differences. 

 

3.2: Probably don’t need to reference the figure each time. I would add a sentence to the beginning that says something like “Results of CPMAS NMR are shown in figure 2. Briefly,…”

 

Discussion

Generally, you lead each paragraph with a result. Also, the trials have fairly complex abbreviation. There’s no inherent problem with this, but it might read better if you at least repeat what the trials have in common that led to the observation you’re interpreting, e.g. “Treatments with crop residues, animal manure, and legume intercropping (CtRML and MtRML) had greater SOC contrent compared to controls.”

 

I would also start off the discussion with a roadmap of your findings. Make a brief statement of the results and one to three major talking points for the discussion. It serves as an introduction and gives the reader an idea of the narrative you’re trying to convey. 

 

L250: High microbial activity can also lead to a loss of C. Can you elaborate more on why you think increased microbial activity led to more SOC in this case?

 

L252: Lignin is generally no longer considered to be “recalcitrant”, and decays quickly under most soil conditions (See Klotzbücher et al., 2011, DOI: 10.1890/10-1307.1)

 

L273: Can you explain what you mean by “distribution trend”?

 

L275: N limitation may have resulted in immobilization, as you say, as well as N mining from SOM, which would reduce SOC. Personally, I would not expect significant microbial oxidation of N without anoxic conditions.

 

L282-284: I don’t follow this sentence. I think the use of “inputs” as both the subject and direct object of the sentence confuses the meaning. 

 

L297: What about these compounds makes them resistant to decomposition? Is it the molecules themselves, or interactions with the mineral phase?

L299: Priming?

 

L303: Switch the clauses: “These alkyl C components have been found to be preserved during SOM decompositions (Citation)”

 

L316: Is this really slower decomposition? Or is it that the inputs offset some of the losses? Actually, the reference to Zhang beginning on line 312 is confusing and I don’t see how it adds to the story. Perhaps it can be re-phrased to tie in with the previous sentence (which is great!)

 

L320: This sentence is a fragment. Either remove “Since”, or add an effect of alkyl C to the end of the sentence. 

 

L338: Uses treatment and treatments one word apart

 

L340: Where does this information come from? What is due to the high fungal bacterial ratio? Needs to be set up more clearly.

 

L370: Specify which ratios are being referred to

 

L372: The authors introduce this ratio (and an abbreviation) and then don’t discuss or apply either. You move on to another ratio two lines later. 

 

L380: Expand on the mechanisms being referred to. Through what mechanisms is it more stable?

 

L392-394: Very nice.

 

L415-420: Tillage is often destructive to soil structure, so I would appreciate more discussion of how tillage preserves C rather than contributes to degradation, if this I what the authors argue. 

Author Response

Thank you for the review. Find attached the responses.

Author Response File: Author Response.docx

Reviewer 2 Report

Overall the article presents a comprehensive overview of the effects of various soil fertility inputs and tillage practices on soil organic carbon (SOC) levels and maize crop performance in the Central Highlands of Kenya. The authors have clearly outlined their research design, questions, hypotheses, and methods, and have used a variety of techniques to analyze the soil samples, including proximate analysis and solid-state NMR spectroscopy. However there are a few areas that can be improved to make the article even stronger:

The introduction section provides a broad overview of the topic of soil fertility and SOC levels, but it could be further contextualized with respect to previous and present theoretical background and empirical research specific to the Central Highlands of Kenya. This would help to establish the significance of the research and provide a clearer rationale for the study.

The Materials and Methods section of the text provided, provides clear information on the experimental design, treatments, and analytical techniques used in the study. The description of the study site, sampling methods, and soil analysis are well detailed and easy to follow. However, the statistical analysis section is very poorly described and needs some serious revision as no explanation is given on the normality of the data or of the homogeneity of variances. It would be beneficial if there was more information provided as well as the criteria for determining the significance of the results. This would help in understanding the robustness of the results and the confidence in the conclusions that were drawn from the study.

The results section presents a lot of data and information, but it can be improved by providing more clear and concise interpretations of the findings and highlighting the key takeaways. In addition, the paper needs some more explanation in their results section. The authors might want to include some suggestions of how their results could be used in practical agriculture and conservation in the future, in order to make the paper even more useful.

In the discussion section, the authors present a lot of arguments and interpretations, but they could be more cohesively integrated with the results and findings. It may be helpful to create a more clear and consistent structure for the discussion, such as grouping related findings together and providing a clear comparison between different treatments.

In the conclusion, the authors could provide some more specific recommendations for how their findings can be applied in practice to improve soil fertility and crop performance in the Central Highlands of Kenya.

Overall, the study provides valuable insights into the effects of different soil fertility inputs and tillage practices on SOC levels and crop performance, and with some minor revisions, it could be an even stronger contribution to the field.

Author Response

Thank you for the review. Find attached the responses.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Thank you for submitting this revised manuscript. It is clear that substantial effort was made to reduce clutter and streamline the message. 

The authors present data from replicated soil amendments to determine the effects on C stocks and C chemistry relevant to various decomposition pathways. This is great, it's good data, and it's an important study. That's why I would really like to see this improved substantially before publication. 

I found that the conclusion describes most of my issues with the manuscript, particularly the discussion. The conclusion succinctly summarizes the results and contextualizes them; I feel that I learned more reading the conclusion than the discussion, due to density of the discussion. I think that the conclusion as written now is a good starting point for the first paragraph as the discussion. 

You have two main results: that organic amendments are key to increasing C stocks, and that mineral fertilizer can promote stabilization. There are of course results about tillage and other amendments that are key to include in the discussion too, but more briefly. I think that the conclusion sets up the two main points of the work and sends the reader off with a clear picture. That's why I think a modified version could be a good launching point for the discussion.

I feel that there are still too many results being repeated in the discussion. I think a framework/introductory paragraph in the discussion would guide the authors and the readers through what is important, and how results support the claims presented in the first discussion paragraph. Please be careful with how you argue for C accumulation; I felt that some of the claims supporting accumulation were alternately used to promote decomposition. Make sure you have a consistent narrative.

Generally, the English language in this manuscript is greatly improved. I think this is a great new draft, but it isn't ready yet. In addition, there were many fragments and typos that could have been caught by another read-through. I have not provided line-by-line comments in the discussion, since I feel that it should be reworked. 

I'm excited to see the finished version; I think this could be a rather impactful citation! 

l55: perhaps "increased soil moisture" or "soil moisture retention"

l67: Try something like "Most or all of this pool is assumed to SOC, except for small amounts of inorganic C in some soils"

l101: "While soil carbon is an indicator..."

l240: "In a nut-shell" is a bit too casual, try "in summary" or something similar instead.

l208, 243, 253: Just make sure to format the horizontal lines before the next submission

 

Author Response

We have attached a document detailing how we have addressed the reviewer's comments for ease of reference.

Author Response File: Author Response.docx

Round 3

Reviewer 1 Report

Thank you very much for working hard on this draft. I think it is a great improvement from the previous draft, and I must commend you for the effort! Well done.

 

There remain numerous small grammatical, formatting, and English mistakes in this manuscript, but I do not think they contribute major distractions. I would suggest a final read-through by a non-associated person to catch some of these bugs. The biggest ones that I noticed are included below.

 

All in all, I think the manuscript communicates the message of the data clearly. I was very happy to see the discussion’s introductory paragraph! I think it can be edited a bit to read smoothly and get the reader excited, but it does a good job of giving a roadmap.

 

Section 4.1 is quite good. I found that it strongly communicates its points, although the phrasing could be cleaned up a bit for readability. Good work! More specific comments below.

 

Sections 4.2 and 4.3 are each too long, however, and are more difficult to read. The structure is still “repeat result, suggest an answer, cite a supporting and/or contradicting study”. I don’t like to repeat results this much in a discussion, but otherwise this structure works a few times. It becomes tiresome to read, although these sections contain some of the most interesting results of this manuscript. Don’t feel that you need to include all of your homework in the paper. A good discussion points to the main findings and ties up a nice narrative. Don’t be afraid to cut and condense these sections to drive home the main points. It’s all important, but there isn’t enough room to interpret your results in every direction. 

 

I think there is a great story here. I’m impressed with the improvements made in this draft, and I look forward to the final one!

 

Line by line comments below.

 

L146: I don’t understand what “land plowing” means. The sentence is hard to follow.

 

L171: use “g” as an abbreviation for grams

 

L207, 241, 251: Still need to fix this blue line to keep it within the bounds of the plot.

 

L253: This introduction paragraph is a huge improvement. I still think it could be streamlined to get at each main finding in 2, maybe 3 sentences, but I’m happy to see this.

 

L281: If you are also implying greater N inputs via legumes, say so.

 

L290: Do you mean that greater enzyme activity is increasing SOC stocks directly? Maybe it would be more accurate to say “stabilizing SOC” or “increasing stable SOC stocks” and then citing a paper that discusses “microbial necromass”. 

 

L293: This sentence is confusing, and you should not refer to C possessing anything.

 

L294: Remove “through increased plant C input”. The way this is written, it implies two unrelated things. If the management increased root growth, then yes, it may have also increased C inputs via exudation or fine root death. 

 

L300: Remove “this” from “attributed this”.

 

L304-306: This sentence is long and hard to follow. I believe the problem is with the “as” on line 305, but unfoI can’t provide a suggestion because I can’t determine what the sentence is trying to say.

 

313-316: Try to fold this reference in somewhere else or take this paragraph out. Or, try to combine this with the next paragraph and write a strong topic sentence to specifically address the topic of tillage, which is an important one in your study. It deserves more attention!

 

L329-331: These two thing contradict one another. If labile C is decomposed more rapidly, then how can it accumulate?

 

L334: I can imply the text which these references are intended to support, but please be careful in the future with consistent formatting before you submit.

 

L384-385: How does this happen? Is it a direct process? 

 

L395: “A study by…”

 

L397-399: This is too vague. Can you give an example?

 

L408: Is this “aliphatic” supposed to be there?

 

L411-413: This sentence is too long. Try to write in a more active voice. 

Author Response

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Author Response File: Author Response.docx