Carbon Benefits and Water Costs of Cover Crops by Assimilating Sentinel-2 and Landsat-8 Images in a Crop Model

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript "Carbon Benefits and Water Costs of Cover Crops by assimilating Sentinel-2 and Landsat-8 images in a Crop Model" focuses on the balance between carbon benefits and water consumption of cover crops in southwestern France. It aims to quantify the biomass, carbon cycle, and water cycle components of cover crops through a hybrid approach combining modeling, remote sensing, and data assimilation (AgriCarbon-EO), and to reveal the trade-off mechanisms between them. The research addresses well-defined scientific questions and employs innovative technical methods with strict validation. This work offers valuable references for agroecological management in water-limited regions.

 

Major Concerns

1. Lack of Quantitative Support for Robustness of Core Conclusions and Vague Uncertainty Boundaries

The impact of model simplifications on conclusions is not quantified.

The authors acknowledge significant model simplifications in the discussion, such as the consideration of only mineralization in soil respiration and the neglect of residue humification and pest dynamics. However, these are merely described qualitatively as "existing biases," without clarifying the magnitude of their impact on core conclusions:

The RMSE of NEE simulations during bare soil periods reaches 1.51 gC·m⁻². Does this error lead to an "underestimation of carbon benefits"? If so, would it change the "positive correlation slope between carbon and water in dry years" from 0.70 mm/gC·m⁻² to 0.50 mm/gC·m⁻², thereby weakening or even subverting the core conclusion that "carbon-water trade-offs are influenced by climate"?

What is the deviation in carbon input estimation caused by ignoring residue humification dynamics? Is the current "annual median carbon input of 77 gC·m⁻²" overestimated or underestimated? If a humification module is added, would the correction range be ±10% or ±30%?

This approach of "only raising issues without quantifying impacts" results in a lack of clear "uncertainty boundaries" for the conclusions, making it impossible for readers to judge the range of errors within which the conclusions remain valid. It is strongly recommended that the authors quantify the impact of model simplifications on core conclusions and explicitly define the uncertainty boundaries to enhance the reliability of the findings.

2. Mismatch Between Research Objectives and Data Support, Insufficient Capture of Interannual Variability

The manuscript explicitly states its objective: to address the gap in traditional studies that "fail to consider the full spatial and interannual variability of cover crops in yield and phenology." However, the data support is mismatched with this objective:

The biomass sampling period is too short: relying only on data from two fallow periods (2018-2019 and 2020-2021), while the study conclusions emphasize "significant interannual differences." The limited sampling period makes it difficult to distinguish between "true interannual trends" and "accidental climate fluctuations," resulting in insufficient long-term data support for conclusions regarding "interannual variability."

It is suggested that the authors supplement long-term monitoring data or conduct scenario simulations to verify the stability of conclusions about interannual variability, ensuring that the data can effectively support the research objectives.

3. Results Analysis Remains at the Level of Correlation Description, Lacking In-depth Mechanistic Interpretation

The study identifies "positive correlations between carbon benefits and evapotranspiration" and "stronger correlations in dry years" but fails to explore the underlying driving mechanisms, leaving conclusions at the level of "phenomenological description":

Failure to distinguish between "direct drivers (crop physiology)" and "indirect drivers (environmental conditions)": Is the positive carbon-water correlation due to "increased biomass to enhanced photosynthesis to synchronized increase in transpiration" (physiological coupling of crops themselves) or "high radiation/temperature promoting both photosynthesis and transpiration" (joint driving by environmental factors)? The contribution ratios of these two mechanisms are not quantified.

Failure to explain the "intrinsic reasons for differences in soil types": The carbon-water slope is steeper in sandy soils. Is this because "poor water retention requires more water consumption to achieve the same carbon benefit" or "lower transpiration efficiency of crops in sandy soils"? The lack of analysis on soil-crop interaction mechanisms makes it difficult to guide targeted management strategies.

It is crucial that the authors deepen the mechanistic interpretation of results, quantify the contributions of different driving factors, and elevate conclusions from phenomenological observations to mechanistic understanding.

Minor Concerns

1. Unclear error allocation in data assimilation

The BASALT method integrates multi-source information such as satellite-derived GLAI and climate data, but it does not clarify the error weights of different data sources. How errors from Sentinel-2 GLAI inversion and ERA5-Land precipitation data are allocated. This may lead to assimilation results being biased toward data sources with larger errors, affecting simulation reliability.

2. Lack of uncertainty visualization in figures

Key figures-Figure 4 for biomass comparison and Figure 8 for carbon-water scatter plots do not show confidence intervals of simulated values, making it impossible to intuitively reflect the reliability of results. It is recommended to add error bars or probability distribution shadows.

3. Insufficient support for regional urgency in the background

The manuscript mentions "limited and decreasing water resources" but lacks specific data, making it difficult to highlight the practical significance of the study. It is suggested to supplement data on regional water resource change trends.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

[1] LINE21: the unit should use · instead of .

[2] LINE98: the introduction is well written with clear logic. It would be better to add some more research significance in the last paragraph.

[3] figure1: it is better to use (a) (b) (c) for subfigures. And the legend of the line or point should also be explained in the figure. Please check and modify the following figure too.

[4] LINE158: when the BASALT first appeared, it should be listed as its full name and be well explained, rather in the following paragraph ( line 305-306).

[5] figure7: the scale and units of the vertical coordinate axis are missing

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript presents a well-designed study using an innovative hybrid approach (AgriCarbon-EO) to quantify the carbon sequestration potential and water trade-offs of winter cover crops (CCs) in southwestern France. The integration of remote sensing (Sentinel-2/Landsat-8) with the SAFYE-CO2 model to generate high-resolution, spatially explicit diagnostics is a significant strength. The validation against eddy covariance fluxes and extensive biomass datasets is robust. However, several methodological limitations, presentation issues, and insufficient discussion of uncertainties need addressing before publication.

1. The abstract section is unclear, and the results are presented broadly.
2. The literature review is too general and thus can't indicate any novelty of the current study. It is better that explain more about the novelty of manuscript in introduction section. The manuscript has not quite innovative. Please explain about its novelty.
3. Compare with several previously published studies. In this case, develop the background of the research more and more fully by mentioning the number of works that are similar to this work.
4. The 250-m resolution soil grid data may not be sufficiently accurate for water flux calculations within the SAFYE-CO2 model; however, this limitation appears to have been overlooked.
5. NDVI>0.4 threshold may exclude low-biomass CCs, but no justification is provided.
6. The tables and figures are not adequate and clear.
7. The discussion and comparison with relevant international studies are limited.
8. Conclusion is long and not clear.so write it shortly with important sentences.
9. Some assumptions are stated in various sections. Justifications should be provided on these assumptions. Evaluation on how they will affect the results should be made.
10. Improve the overall coherence of the manuscript by avoiding short paragraphs and ensuring a smooth flow of ideas. Revise language in certain sections to improve clarity and readability. By addressing the revisions suggested, the scientific quality, clarity, and coherence of the manuscript will be enhanced, making it more suitable for publication.
11. Number of old references need to decrease and use 2017-2025 references.
12. The major defect of this study is the debate or Argument is not clear stated in the introduction session. Hence, the contribution is weak in this manuscript. I would suggest the author to enhance your theoretical discussion and arrives your debate or argument.
13. It is suggested to compare the results of the present research with some similar studies which is done before. Much more explanations and interpretations must be added for the results, which are not enough.
14. Please make sure your conclusions' section underscore the scientific value added of your paper, and/or the applicability of your findings/results, as indicated previously. Please revise your conclusion part into more details. Basically, you should enhance your contributions, limitations, underscore the scientific value added of your paper, and/or the applicability of your findings/results and future study in this session.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript "Carbon Benefits and Water Costs of Cover Crops by assimilating Sentinel-2 and Landsat-8 images in a Crop Model" focuses on the balance between carbon benefits and water consumption of cover crops in southwestern France. It aims to quantify the biomass, carbon cycle, and water cycle components of cover crops through a hybrid approach combining modeling, remote sensing, and data assimilation (AgriCarbon-EO), and to reveal the trade-off mechanisms between them. The research addresses well-defined scientific questions and employs innovative technical methods with strict validation. This work offers valuable references for agroecological management in water-limited regions.

 

Major Concerns

The discussion section of the article is overly weak, causing the results to remain at the stage of data description. In fact, the original text cites relevant literature in multiple places, but these citations are very scattered and fail to be integrated into a complete logical explanation. Although key data are presented, it fails to deeply combine "what is" with "why" through mechanism integration and quantitative analysis, which weakens the explanatory power of the discussion section for the results.

For example, regarding the two aspects of the positive correlation between cover crop carbon input (dSOC) and evapotranspiration (ETR) as well as their physiological mechanisms, and the mitigation of cover crops' impact on soil moisture by spring precipitation, the original text reflects them through data and fragmented information. However, neither the "Results" nor the "Discussion" sections have carried out systematic integration and mechanistic explanation. The specific manifestations are as follows:

Positive correlation between carbon input and ETR and their physiological mechanisms:

The "Results" section clearly presents the positive trend between dSOC and ETR, with slopes of 0.11-0.28 mm per gC·m⁻² from 2017 to 2019 and 0.51-0.70 mm per gC·m⁻² in 2020-2021. It also mentions that the Green Leaf Area Index (GLAI) is related to photosynthesis and transpiration, where photosynthesis involves carbon fixation, transpiration is a component of ETR, and biomass growth is also related to GLAI. However, the "Results" do not link these data to physiological mechanisms, and the "Discussion" does not further explain the complete logic, merely staying at the description of trends without in-depth analysis of the intrinsic physiological processes underlying the correlation between the two.

Mitigation of cover crops' impact on soil moisture by spring precipitation:

The "Abstract" and "Conclusion" directly point out that "due to spring precipitation, the impact of cover crops on soil moisture is minimal". The "Results" section also mentions that the difference in deep soil moisture among years is small, with a median of approximately 0.27 m³·m⁻³. However, the "Results" do not combine spring precipitation data to specifically explain how precipitation supplements soil moisture and offsets the moisture consumption by cover crops; the "Discussion" also fails to conduct mechanistic analysis on this phenomenon, failing to clarify the quantitative relationship between spring precipitation and soil moisture balance and the mitigation path for cover crops' moisture competition.

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have carefully revised the manuscript in accordance with the first-round review comments. Relevant literature has been supplemented, the description of research methods has been improved, and the data analysis and discussion sections have been refined and logically reorganized. The language expression has also been noticeably improved, and the formatting and figures/tables now conform to the journal’s requirements.

Upon review, the revised manuscript has addressed the previous concerns well, the research conclusions are now more rigorous, and the overall argumentation is more comprehensive. The overall quality has been significantly enhanced. I recommend that the editorial office accept the paper or proceed with only minor editorial adjustments prior to publication.

Author Response

Thank you very much for taking the time to carefully review our study and for your valuable comments.