AquaCrop-Simulated Response of Sorghum Biomass and Grain Yield to Biochar Amendment in South Sudan

Round 1

Reviewer 1 Report

Dear Author(s),

The manuscript Agronomy_644450 presents a properly conducted valuable research based on AquaCrop simulation of sorghum biomass and
grain yield after biochar amendment in South Sudan. This paper discusses results of model simulations using AquaCrop. The topic investigated, and the approach proposed are original and innovative. The novel approach reported in the manuscript is the use of simulation model as a valuable tool to assess the interactions of management practices and soil water moisture content. It is a straight-forward analysis, which can be useful for making predictions and assessing the impacts of different management on biomass yield and soil water balance. The title is clear and explicative. The abstract is concise, and the aim is well explained. The literature background interests the reader on the importance of the topic and the significance of work is clear and well explained. The Materials and Methods section is informative, but some changes are required. I suggest to better explain the calibration and validation procedures, and add the model performance section reporting key statistical parameters to compare modelled and observed data. Findings are new and extremely informative for future research in similar environment and topic, but validation and model performance must be better described and discussed. Results and discussion section is well-written. In the conclusions section, the main findings are presented in an effective way. However, I suggest to propose a take-home message which include information for policy-makers, stakeholders, and decision-makers. The Tables and Figures are informative, and they have a fully self-explanatory caption. This manuscript presents interesting research data, with important implications for future investigations in similar environmental conditions at local and global level.

 

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

General comments:

The manuscript of Deng et al. presents an original approach to simulate the effect of biochar amendments on Sorghum production, focusing on the hypothesis that such an effect would mainly occur because of an improved soil water retention during dry years. Besides some detailed comments listed below, I have some general remarks about this work.

I am not completely convinced by the robustness of the findings, as there are no experimental data from the year 1990 to support the findings that biochar amendments on this site can indeed improve the yield in dry years, and that this improvement in yield would be linked to an improvement in soil water retention only. This conclusion is only based on simulations with a model that focuses on water fluxes, but that seems to neglect any other processes controlling nutrient availability. The fact that a “soil fertility stress” was needed to fit the simulations with biochar to the experimental data in 2011 and 2012 actually suggests that predicting improvements in soil water retention is not enough to predict Sorghum yields.

Furthermore, the simulations of biochar effects on soil water retention are themselves not based on actual experimental data obtained from the soil at this site, but only on some general results from a meta-analysis, which was likely derived from data obtained in different pedoclimatic conditions. The simulations would be more convincing if the changes in soil hydrological parameters were actually measured on the soil samples from the investigated site after biochar amendments. The conclusion might also be strengthened if several dry years were analyzed, and not only the year 1990.

Eventually, it seems to me that this work simply corresponds to a modelling exercise, where an existing water-focused model was used to speculate about the use of biochar in this particular context, but I am not sure then how strong the conclusions can be. I am not opposed to the publication of this work, but I think authors should better explain (maybe with a scheme?) which simulations are derived from experimental data and which are not, discuss all possible limitations of their approach, and eventually balance their overall conclusions when considering these limitations.

Detailed comments:

Abstract:

L15 : I am not sure that biochar technologies are considered « relatively cheap » by the whole community, although I understand that in this work biochar was produced in traditional kilns, without gas recovery. I would remove this from the abstract.

L21: Measured with or without biochar amendments?

L21-22: “Changes in… study.” I suggest to move this sentence at the end of L19, before mentioning the effect of biochar for a dry year.

L23: Rephrase “rainfall amount and variability”

L23: Please be specific about the fact that these are simulations. “Simulations showed that biochar amendments had an effect”

L25-27: It’s an interesting statement but it is not at all discussed in the manuscript, only mentioned in one sentence at the end of conclusion

Introduction:

L44: Please give a definition of biochar

L66 and 70: bought about?

Material & Methods:

L111: phenological?

L114: Shouldn’t root penetrability also be affected by biochar amendments?

L118: brought?

L148-150: This information is already given L128-130

L151-154: If I understand correctly, inclusion of a soil fertility stress was necessary in the case of biochar-amended soils but not in the control soils, so that the simulations with biochar amendments can match the experimentally determined data of plant production in 2011 and 2012. This is important and should be better explained and justified.

L169: overlying?

L173: “the range in reported percentage changes in hydraulic properties”. I don’t understand what this means and how the 4 different combinations of biochar-modified parameters were actually chosen.

L178: remove “and”

L207-208: Please avoid using abbreviations that are not trivial to remember. Use “plant wilting point” and “field capacity” instead.

Discussion:

L259-260: “we found a direct relationship between yield and rainfall”. If this observation is only based on simulations with AquaCrop model, it is not very convincing since this model is based on the hypothesis that soil water dynamics is driving the yield…

L271: Rephrase “to be able successfully simulate”

L275-276: “can be reasonably considered to be valid”. Why can it be considered so? This paragraph does not mention anything about biochar…

L305: Need rephrasing

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

General comments:

The manuscript by Deng et al. has been significantly improved in this revised version, and I am satisfied with the answers provided by the authors to my detailed comments.

I am less convinced by the answers to my general concerns. In particular, while I think this work is valuable and brings useful new information, the conclusion that biochar may indeed help to increase crop yield in dry years is only based on simulations (not verified by experimental data) obtained with a model that appears to focus on water fluxes. But there are several mechanisms why biochar could (or could not) modify crop yield, as noted by the authors in the Introduction and as listed in the meta-analyses mentioned there. Authors should therefore be aware that one of their implicit hypotheses is that in this particular context of Vertisols in Sub-Saharan Africa, biochar would be expected to affect yield only through its effect on soil water dynamics, and not through other mechanisms (for example release or immobilization of nutrients, release of other chemicals, soil aeration, changes in albedo and soil temperature, etc.). This is a perfectly acceptable hypothesis, but it should be clearly stated in the Introduction, and maybe revisited later on in the Discussion. In general, I keep thinking that this work would benefit from a discussion about the possible limitations of this modelling approach, and how this could be improved in future works.

Minor comments:

L24 (or L29): Authors should explicitly say in the abstract that 1990 corresponds to an “extremely dry” year as stated in the Introduction, so that the reader understands why this particular year was also investigated and why it led to distinct results.

Author Response

Reviewer 2 second round

 Comments and Suggestions for Authors

General comments:

The manuscript by Deng et al. has been significantly improved in this revised version, and I am satisfied with the answers provided by the authors to my detailed comments.

I am less convinced by the answers to my general concerns. In particular, while I think this work is valuable and brings useful new information, the conclusion that biochar may indeed help to increase crop yield in dry years is only based on simulations (not verified by experimental data) obtained with a model that appears to focus on water fluxes. But there are several mechanisms why biochar could (or could not) modify crop yield, as noted by the authors in the Introduction and as listed in the meta-analyses mentioned there. Authors should therefore be aware that one of their implicit hypotheses is that in this particular context of Vertisols in Sub-Saharan Africa, biochar would be expected to affect yield only through its effect on soil water dynamics, and not through other mechanisms (for example release or immobilization of nutrients, release of other chemicals, soil aeration, changes in albedo and soil temperature, etc.). This is a perfectly acceptable hypothesis, but it should be clearly stated in the Introduction, and maybe revisited later on in the Discussion. In general, I keep thinking that this work would benefit from a discussion about the possible limitations of this modelling approach, and how this could be improved in future works.

We thank R2 for these additional comments to our manuscript. Given the constraints of the existing manuscript, we have revised the text in accordance with the comments and hope they are acceptable.

L51-53: Replaced the first part of the sentence with "The main benefit of applying biochar in SSA [14], where much of the agriculture is rainfed, may be expected to be its effect on increasing the water holding capacity of the soil, but ...".

 

L315: Added this sentence to beginning of 3rd paragraph of the Discussion. "Although biochar can affect crop yields through a number of mechanisms, for example release or immobilization of nutrients, release of other chemicals, soil aeration, and changes in albedo and soil temperature [10], yields in SSA are probably mostly affected by its effect on soil hydraulic properties, particularly on water holding capacity."

 

L377: Added the following last sentence to Conclusions. "However, given the inherent limitations of the modelling approach, further empirical studies on the effects of biochar on crop yields in SSA, especially in view of expected climate change impacts on the water cycle, are needed."

 

Minor comments:

L24 (or L29): Authors should explicitly say in the abstract that 1990 corresponds to an “extremely dry” year as stated in the Introduction, so that the reader understands why this particular year was also investigated and why it led to distinct results.

L24-25: replaced end of sentence with "...whether the response to biochar differed in dry years, simulations were also carried out for 1990, which was the driest year during 1979-2014."

Author Response File: Author Response.docx