the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Sep 2024
Depth Effects of Long-term Organic Residue Application on Soil Organic Carbon Stocks in Central Kenya
Abstract. In arable soils, a substantial portion of soil organic carbon (SOC) is stored below the plough layer. To develop sustainable soil management strategies, it is important to assess how they affect the quantity of SOC stored in the subsoil. Therefore, we investigated the impact of organic and inorganic nutrient inputs on SOC stocks down to 70 cm depth in a long-term field trial in Embu, Kenya. There were 3 organic input treatments (manure, Tithonia diversifolia residues, and maize stover) and a control treatment, each with and without the application of mineral nitrogen. These different treatments were applied to a maize monoculture over 38 growing seasons (19 years). Our results show that manure application had the largest positive impact on SOC stocks compared to the control, which was observed down to 60 cm depth. In contrast, Tithonia diversifoliaand maize stover significantly increased SOC compared to the control only within the top 20 cm and 40 cm, respectively. Among the three organic residue treatments, only the application of manure had a significant effect on the SOC stock of the subsoil (i.e., the 30–70 cm depth layer). However, when considering the whole measured profile (i.e., 0–70 cm), all treatments led to significantly higher SOC stocks compared to the 91 ± 12 t C ha-1 of the control: manure had the highest stocks (120 ± 24 t C ha-1), followed by maize stover (112 ± 17 t C ha-1) and tithonia diversifolia (105 ± 11 C t ha-1). Mineral nitrogen application did not have a significant impact on SOC stocks down to 70 cm depth. Overall, our findings indicate that the subsoil in the studied agroecosystems is affected by the type of added organic amendments. Our results imply that gathering knowledge on the soil below the typically studied 0–30 cm depth layer will improve the overall assessment of agroecosystem properties, which is necessary to optimize soil system resilience, limit organic matter losses and improve crop productivity.
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Status: open (until 20 Nov 2024)
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RC1: 'Comment on egusphere-2024-2796', Anonymous Referee #1, 18 Oct 2024
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This paper represents a nice study of the impacts of different forms of agronomic management (w/ and w/o N amendment & 3 different organic residues) on deep carbon stocks. As the paper highlights, this is an area that is in need of more study because most studies focus on the top layers of soil, whereas we know that much carbon, and especially stable carbon is in deeper soils. My biggest comment is on the statistical analysis, and potential interpretations. I applaud the authors for conducting a power analysis and being quite clear about where there are limitations on sample numbers. That said, I think there may need to be a little more caveating of the results earlier on in the study, and also an indication that there is also the potential for type I errors as well (I believe...). I'm also slightly concerned about model over-fitting with the inclusion of all of the interactions, given the number of samples present in the study. Especially given that almost none are significant, and also I don't think there is sufficient justification for inclusion of all of the interactions - ideally, the linear model should include interactions with sound theoretical justification and hypotheses around why these interactions might reasonable shift overall carbon levels, and specifically deep carbon. An additional analysis that only subsets deep carbon may also be interesting to show, given that this is the piece that is particularly novel for this study. I presume that much of the presented model is driven by the carbon in the top soil. Some level of sensitivity analysis would also help make me more confident in the results of your model - e.g. do model results stand when you include fewer variables/interactions. This could be presented in supplements. Finally, I think this study should be published, but the language around your findings should be slightly less 'confident' given the results of your statistical analysis - this could just be done earlier on.
I'm a bit confused on the calculation of stocks given that I think you only measured a subsets of the soils at depth - in line 165 you say that you look at layers of 5cm increments, but these do not add up to the whole soil profile - do you assume that 45-50 is representative of 40-50? Perhaps I missed this explanation, but I think there should be some statement that makes clear what assumptions you are making on the calculation of stocks.
The intro covers some good ground but in its current form read a bit choppy. The isotope paragraph feels like its just floating and is not needed in the intro. The flow between paragraphs could also use a bit of work. A bit more context for the specific system could be useful in the final paragraph with the research questions. I also think that these questions still read somewhat broad, and contextualizing them to your system can add some useful specificity.I personally also think its nice, even if briefly, to state what you expected out of these different schemes given the current literature.
I'm not totally convinced that the isotope work fits in currently in a way that feels meaningful. The final data is somewhat mixed in its results and while I think its interesting, I'm not sure that in its current form that it is adding to the manuscript or primary findings. I think the paragraph in the intro feels quite plopped in, and while I'm not against its inclusion, the authors should work to make its inclusion feel less like an add-on.
The following are more specific/smaller comments that I think would strengthen the manuscript:
33: citation for ...groundwater and the atmosphere; "it" should be clarified - I presume it is something like "intensive agriculture" or something?
41: OC is not defined in its first use.
42: "This is clear from the fact" is unnecessarily wordy
49: Are there key functions other than nutrient management that you think are relevant to bring up? What are the key functions
66-67: "However" not "Nevertheless" works better; I don't think you have defined OR - I presume organic residue.
92: Not sure about this paragraph and how it fits in.
115: I'm not experienced in this region, but I'm a bit confused about the rain seasons - is it raning all year round? the given months span the entire year.
117: passive voice not needed "having been" => " the site was originally covered... before conversion to" would be clearer.
125: Would be good to clarify that these treatments are the same as in this paper - this whole paragraph maybe feels like it should be in the intro justifying the gap in the literature of deeper depth investigations. This paper seems like a fundamental work that you build off of.
139: Are these typical application rates for the region? Also some justification of Thitonia, corn stover, and maize (which I presume has happened in previous papers) would be good to include in brief somewhere. Perhaps in Study Area. Jumping back to "The organic resources" is a little confusing after discussing other amendments. Also, are organic resources = organic residues as you've referred to them before. Consistency in terminology would be good.
Para at 146 is not needed.
171: no need to repeat. Decide where and how you want to organize without repetition from above paragraph.
182: HCl DID or DID NOT show a reaction? If there is a reaction, I would think this means there ARE carbonates?
205-207: Extra line?
256: "THis was only the case for...: and 267 "The results indicated..." should be in results, not methods.
273-274:Cohen's d - "d" should be ital. "Significance" not "Significant"
287: MIneral N fertilizer - spelling inconsistency
Figure 3: the overlapping error bars make it pretty hard to read... any ways to clean this up or make it more legible would be good.
387: Unclear when you are/aren't italicizing
392: Given the immediately preceding statement on your power analysis, I wonder if you could rephrase here to bring your conclusion in better alignment with your actual statistical tests.
449: "Very" would be better than "much responsive"
476-481: "Therefore, although there might be..." This sentence is too long - break it up. I'm also a bit weary of the sudden introduction of the co2 sequestration piece. Might be clearer to just keep it to your context of increasing SOC to offset losses in this cropping system. Some statement on whether you think there's any ways to actually stabilize or rebuild SOC such that there isn't continual losses would be interesting to me as well... how much of this is the continual maize?
Citation: https://doi.org/10.5194/egusphere-2024-2796-RC1
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Depth Effects of Long-term Organic Residue Application on Soil Organic Carbon Stocks in Central Kenya Claude Müller https://figshare.com/s/c2f2787b7a56ef7ad656?file=49027291
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