Responses of Soil Organic/Inorganic Carbon Concentrations in the Lower Yangtze River to Soil Development and Land Use

Response to Reviewer 1 Comments

Thanks very much for the reviewer’s comments, which improved our manuscript a lot. We have fully revised this manuscript according to the comments. We make a point-by-point reply to these comments as below. Please feel free to contact us if further revisions are required.

The reviewed work raises important issues concerning the role of soil in the storage and release of C. The results presented in the work can be used in sustainable soil management.

R: We sincerely appreciate your support for this study and your encouragement. Your suggestions have been invaluable in enhancing the quality of our research, particularly your insights on how to optimize the standardization and hypothesis. These contributions have deepened our understanding of the topic's significance and improved our writing skills. We have revised the manuscript according to your recommendations and marked the changes in red, hoping to meet your expectations. Thank you again for your review.

2. Point-by-point response to Comments and Suggestions for Authors

Q1: Line 40- …’ and topography—can directly alter the ratio and quantity of OC and IC [12-13],…’ please explain the abbreviations OC and IC

R: Thank you for your suggestions. The abbreviations OC and IC have been provided as required. (Please see L31 in the revised version)

Q2: Line 206 (Figure 2) – please explain the abbreviations OC and IC in the caption under the Figure

R: Thank you for your suggestions. The abbreviations OC and IC under the Figure have been provided as required. (Please see L155 in the revised version)

Q3: Line 221 (Figure 3) - please explain the abbreviations OC and IC in the caption under the Figure, the same in the case of Figure 4 (line 238) and Figure 6 (line 267)

R: Thank you for your suggestions. The abbreviations OC and IC under Figure 3 and 4 have been provided as required. In particular, Figure 6 has been revised into a table in accordance with the comments provided by Reviewer 2. (Please see L167, L183 and L208-210 in the revised version)

Q4: There is no reference to hypothesis 2, concerning forest soils, in the conclusions. It may be worth considering rewording this hypothesis and putting forward two hypotheses: one concerning rice fields and the other concerning the areas adjacent to them. The Authors are really considering OC and IC in irrigated and dry areas (rice fields and adjacent areas)

R: The “Hypotheses” has been revised as required and has been updated to include two hypotheses that are more closely aligned with the key focus of this study. (Please see L67-73 in the revised version)

Response to Reviewer 2 Comments

We sincerely appreciate the reviewer's insightful comments, which have significantly enhanced the quality of our manuscript. In response to the valuable feedback, we have thoroughly revised the manuscript accordingly. Below, we provide a detailed point-by-point response to each comment. Should any additional revisions be necessary, please do not hesitate to contact us.

The authors present very interesting material – over 1000 sites with marked organic and inorganic carbon contents, the degree of weathering of soil material – sampled both from surface soil horizons and from the 150–180 cm layer (parent material). The obtained results are very interesting and provide great possibilities for interpretation. It is particularly noteworthy that the samples were taken from soils used for agricultural purposes (drylands and paddy fields) as well as from woodlands. Unfortunately, considering the diversity of soil material only in the context of basic soil properties and their use, in isolation from soil genesis (CIA is not enough to explain this), has led to completely (in my opinion) erroneous conclusions. First of all – most soils are formed from autochthonous material – already a product of changes in soil-forming processes – which is why it cannot be considered as a typical chronosequence (it is just more like stage (not age) of pedogenical development of soil material)– especially in comparison to soils from drylands and woodlands. To sum up, the obtained diversity of results certainly does not result only from different forms of use and “age” (which is not real age – because it is allochthonous soil material)  of individual soils. For this reason, a significant part of the conclusions is formulated incorrectly. Moreover, in the woodlands and drylands groups, soils can be significantly differentiated not only because of the different advancement of soil-forming processes but also because of significant differences in parent rocks. A significant problem is the selectivity of the literature used - the authors provide almost exclusively Chinese studies, while the world literature very often addresses the issues of the influence of methods of use, age, climate on the variability of humus resources. In the case of alluvial - allochthonous soils under the influence of flowing water (which still supplies various dissolved compounds - including metal elements - even after sediment deposition), both the IC and OC content cannot be interpreted as a derivative of time or the state of soil development or even the parent material.

Moreover use of CIA for soils of different genesis and made of different compositions is not correct. Unfortunately, the authors did not provide any data on this subject – so there is no way to verify these (serious) doubts. For this reason, I believe that the material itself is very valuable – however, it requires a very thorough re-editing in terms of concepts and interpretations of results and conclusions.

Some detailed comments are listed below (except results and conclusion – these chapters should be changed substantially what was explained above):

R: We sincerely appreciate your support and encouragement for our research. Your suggestions have been invaluable in enhancing the quality of our study, particularly your insights into accurately describing sample sources and optimizing the discussion section. These contributions have deepened our understanding of the topic's significance and improved our writing proficiency. We have revised the manuscript according to your recommendations, with changes highlighted in red, hoping to meet your expectations. We extend our gratitude once again for your thorough review.

Regarding the soil samples used in this study, extensive geological surveys and source analyses were conducted prior to sampling. We strictly ensured that all soil samples were derived from Yangtze River alluvial deposits. Samples from areas with parent materials such as rock weathering, Late Pleistocene loess, clastic rock weathering materials, and vermicular laterite were systematically excluded to maximize the homogeneity of the sample parent materials. This rigorous selection process provides a theoretical foundation for using the Chemical Index of Alteration (CIA) as an indicator of soil development stages. While we acknowledge your perspective that this method may not be entirely reliable, we plan to incorporate quantitative soil age determination in subsequent studies and would greatly value your further guidance on this matter. Additionally, in response to your comments, we have supplemented the manuscript with relevant world literature to enhance the reliability of our research evidence.

2. Point-by-point response to Comments and Suggestions for Authors

Q1: Line 37 - the explanation of these abbreviations appears in the abstract, but I think that in the main text - at their first appearance - they should also be explained - OC - organic carbon, IC - inorganic carbon);

R: Thank you for your suggestions. The abbreviations OC and IC have been provided as required. (Please see L31 in the revised version)

Q2: Line 46 - this assumption is not entirely true. Most of the parent materials of the soils of postglacial areas and the sea coast are acidic rocks with a very low IC content;

R: Thank you for your suggestion. We greatly appreciate your professional guidance regarding soil types. We initially considered that the parent materials in these areas might contain substantial carbonates, leading to higher IC content due to low weathering and leaching in the early stages of soil development. In the revised manuscript, we have removed this inappropriate example.

Q3: Line 56 - what does it mean = Radioactive heat sources? energy from Earth? Do You mean significant amounts of solar energy conditioned by climate? This needs to be written in a clearer way.

R: Thank you for your suggestions. The term "heat resources" here solely refers to high temperatures. We have made the necessary revisions to prevent potential ambiguity. (Please see L44 in the revised version)

Q4: Line 56 - "and" - unnecessary word - delete it;

R: Thank you for your suggestions. The original “and” has been deleted. (Please see L44 in the revised version)

Q5: Do You mean Dryland and temporary conversion drylands to paddy fields? Is it similar to this phenomena described here?:  https://www.sciencedirect.com/science/article/abs/pii/S0341816223003946

R: We sincerely appreciate your suggestion and the provided reference. We would like to clarify that the paddy-dryland rotation in our study differs from that described in the referenced literature. The paddy-dryland rotation in the present study is a typical agricultural practice in southern China (e.g., Jiangsu, Anhui, etc.). Due to climatic constraints, this region does not support year-round rice cultivation. Therefore, rice is grown from April to October each year, while dryland crops such as wheat and rapeseed are cultivated during the remaining months.

Q6: Line 80 - do You mean - lower state of development of soil profiles? minor alternations by pedogenesis?

R: Thank you for your suggestion. We aimed to convey the initial stages of soil weathering and development. To avoid ambiguity, we have made the necessary revisions. (Please see L61 in the revised version)

Q7: Line 117 – figure 1 - the colors of individual places on the map are poorly distinguishable - it is better to make on the map not only the fill but also the outlines of individual dots in the same color, e.g. red (and not only the red fill and the black outline);

R: Thank you for your suggestion. Figure 1 has been revised according to your recommendations. (Please see L89 in the revised version)

Q8: Line 119 - in the case of these studies, where most of the soils are paddy soils, this type of index is of little use - these soils contain an admixture of allochthonous material (and also elements/chemical compounds precipitated from flowing water).

R: Thank you for your suggestion. Your advice is highly professional, as paddy soil is indeed a special soil type with complex provenance. Citing references related to paddy soil here may not be sufficiently persuasive, so we have removed the inappropriately cited references. It is worth noting that the retained reference is from a previous publication by our research team. In a representative area of our study region, we conducted another study where, despite the diversity in land use types, a significant correlation between the CIA and the actual soil age was still observed. Therefore, we retained this reference to support the findings of the current study. (Please see L92-97 in the revised version)

Q9: Line 127 - this formula - align to the left

R: Thank you for your suggestion. This formula has been aligned to the left (Please see L98 in the revised version)

Q10: line 181 - in the case of pH, it makes no sense to give a value to two decimal places. One is enough.;

line 192 – table 1 - t does not make sense to provide this data for the entire set of all locations. At this stage it should be separated into 1) woodland, 2) dryland, 30 paddy-dryland rotation;

R: We sincerely appreciate your suggestion. We would like to address these two questions collectively. We agree with your observation that an overall analysis of the soil data across the entire study area may lack sufficient significance. Therefore, we have conducted separate statistical analyses for different land use types. However, it is important to note that due to the large number of soil indicators (3 × 11) for all land uses, it is challenging to present all the data in a single table. Consequently, in the new table, we have focused on the primary variables in this study—soil organic carbon (SOC) and soil inorganic carbon (SIC)—and provided their statistical summaries. (Please see L144 in the revised version)

Q11: line 222 – fig. 3 - there is no reference to this fig. in the text. The figure is hard to understand - what do C55, C60 etc. mean? If it is age, what does it mean?

Line 229 - there is no reference to this fig. in the text. The figure is hard to understand - what do C55, C60 etc. mean? If it is age, what does it mean?

R: Thank you for your suggestion. Fig 3 has been referred in the text. The X-axis, ranging from C55 to C85, represents the chemical index of alteration of 55, 60, 65, 70, 75, 80, and 85, respectively. Since the soil parent material in this study is limited to uniform alluvial deposits, the soil weathering index is used here to distinguish the gradient stages of soil development. We have also provided an explanation in the revised manuscript. (Please see L159, 167-168, 217-218 in the revised version)

Q12: 4 – line 241 - Do the different colors (green, yellow, blue) indicate different types of use? If so, this should be in the legend.;

R: Thank you for your suggestion. The different colors indeed represent distinct land uses. We have clarified this in the revised legend. (Please see L182 in the revised version)

Q13: 6 – line 271 - This diagram is completely illegible. I suggest we abandon it and replace it with a correlation table.

R: Thank you for your suggestion. We agree with your suggestion. The figure has been replaced with a correlation coefficient table, and the corresponding annotations have been revised accordingly. (Please see L209 in the revised version)

Q14: Line 277 - What do the authors understand by the term development of soil parent material? In terms of soil formation, parent material is always completely unprocessed by soil-forming processes (with the exception of allochthonous colluvial and alluvial material).

R: Thank you for your suggestion. Your advice is absolutely correct. Using the term "development of soil parent material " is not professional, as soil parent material refers to completely unweathered and undeveloped material. Therefore, we have revised this expression in the revised manuscript. We sincerely appreciate your meticulous and rigorous suggestion. (Please see L216 in the revised version)

Q15: Line 294 - Since a significant part of the studied soils is allochthonous, this type of approach is incorrect.

R: Thank you for your suggestion. We fully understand your concerns regarding the applicability of our research method to a single parent material. Indeed, it is challenging to ensure completely identical parent materials across such a large scale and extensive sampling. However, we affirm that prior to soil sampling, we conducted thorough geological background and provenance analyses. Samples originating from rock weathering, Late Pleistocene loess, clastic rock weathering materials, and vermicular laterite were excluded to ensure that all sampling sites derived from Yangtze River alluvium. Based on this premise, we constructed the developmental gradient using the Chemical Index of Alteration.

Response to Reviewer 3 Comments

We sincerely appreciate the reviewer's valuable comments, which have significantly enhanced the quality of our manuscript. We acknowledge that there were indeed shortcomings in the scope of the study, the expression of core content, and the refinement of research objectives. We have diligently revised the manuscript in accordance with the reviewer's suggestions. We provide a point-by-point response to the reviewer's comments. Should any further revisions be necessary, please do not hesitate to contact us.

There is a lack of novelty in the research, the authors try very hard to prove otherwise by introducing beautiful graphs.

The article does not define the research gap, there is no clearly defined research goal. Too local research.

R: We sincerely appreciate your comments of this study. We have revised the introduction to more clearly articulate the research gap this study aims to address. The primary focus of this research is to explore the consistent and divergent evolution of organic carbon (OC) and inorganic carbon (IC) in both surface and deep soil layers along the increasing stages of soil development in the lower Yangtze River basin. Additionally, we examine whether land use intensifies or mitigates this evolutionary effect. We have consolidated the research hypotheses, refining the original three hypotheses into two more specific ones to better clarify the study's objectives. This study is indeed regional in scope, concentrating on soil carbon dynamics within a limited area of the lower Yangtze River, we aim to present it as a model for watershed-scale research to enhance the significance of our findings. This has been rearticulated in the introduction and conclusion section.

Response to Reviewer 4 Comments

We sincerely appreciate the reviewer's insightful comments, which have significantly enhanced the quality of our manuscript. We have thoroughly revised the manuscript accordingly and provided a point-by-point response to each comment. Should any additional revisions be necessary, please do not hesitate to contact us.

Authors studied more than one thousand soil samples in a large area near Yangtze River. This collection enabled authors to reach interesting results regarding evolution and changes in organic and inorganic carbon contents in different soil natural or management systems. The manuscript has high quality, it is very interesting and brings new information. I have same technical comments mentioned in Specific comments.

R: We are deeply grateful for your support and encouragement throughout this study. Your constructive suggestions have been instrumental in elevating the quality of our research, especially your insightful recommendations on optimizing sampling and mapping. These contributions have deepened our understanding of the topic's significance and improved our writing skills. In accordance with your guidance, we have revised the manuscript, with all modifications highlighted in red for your convenience. We sincerely hope that these revisions align with your expectations. Once again, we extend our heartfelt thanks for your thoughtful review.

2. Point-by-point response to Comments and Suggestions for Authors

Q1: Line 107 was it really October – November 2003? Please, confirm. Naturally, it is possible to understand that elaboration of so great number of soil samples need aa lot of time, but it is more than 20 years ago.

 R: Thank you for your valuable suggestions. We confirm that the sampling for this study was conducted between October and November 2003. While it is true that this sampling period is considerably distant from the present, our research team believes that the advantage of this study lie in the large amount of soil samples collected, the uniformity of their parent material origins, the inclusion of different soil layers, and the comprehensive analysis of the samples' geochemical properties. These aspects ensure that the dataset retains certain research value for the accurate and systematic analysis of soil carbon cycling within the watershed. Despite the time elapsed, the data has been well-preserved, and the comprehensive dataset of soil environmental factors ensures the reliability of our analysis regarding soil carbon distribution and its environmental drivers.

We acknowledge that more recent data may provide additional insights as time progresses. Therefore, we are planning to collect updated soil samples and intend to incorporate more contemporary data in future studies to further validate and expand upon our findings.

We fully understand your concerns regarding the timeliness of the data and hope that the above explanation clarifies our rationale for utilizing the 2003 dataset. We are open to further discussion, as per your suggestions, on how to integrate more recent data in future research to enhance the timeliness and relevance of our work.

Q2: Line 110, why the layers between 20 – 150 cm were not analysed. They could bring an interesting data.    

R: Thank you for your valuable suggestions. This study takes deep layer samples as a control group and background values for surface (0-20 cm) soil. Deep layer (150-180 cm) is largely unaffected by external environmental factors and human activities during its development, making it representative of the initial nutrients of the parent material. This sampling design aligns with the specific focus of soil development, and further details are provided. The determination of the 150-180 cm was made based on preliminary investigations of the geological conditions in the study area, which is approximately close to the bedrock depth. Additionally, due to the substantial number of samples, this sampling approach also considers cost-effectiveness.

Q3: Soil ages – how were they determined?

R: Thank you for your valuable suggestions. Since all soil samples in this study are limited to alluvial parent material, the uniformity of the parent material allows us to construct a relative soil development sequence by measuring the chemical index of alteration (CIA). The term "soil age" here actually refers to the gradient of CIA. Considering your comment, combined with Reviewer 2's suggestion, we agree that the use of "soil age" is inappropriate. Therefore, we have made the necessary revisions accordingly.

Q3: Figure 3 Why negative numbering is used in the Y axis? It seems to be strange if units are g/kg. Minus levels seems to be a nonsense.

R: We appreciate your valuable suggestions. This phenomenon is typically attributed to the kernel density estimation component of the violin plot. Kernel density estimation is a non-parametric statistical method used to estimate the probability density function of a random variable. When generating violin plots, kernel density estimation applies smoothing to the data and may extend beyond the actual data range. Consequently, the kernel density plot may produce areas below zero when constructing violin plots. This does not indicate the presence of actual negative values but rather signifies a lower data density in that region. To avoid potential misinterpretation, we have revised the figure to a bar chart. (Please see L166 in the revised version)

Q4: Figure 3 Explain X axis

R: Thank you for your suggestion. The X-axis, ranging from C55 to C85, represents the chemical index of alteration of 55, 60, 65, 70, 75, 80, and 85, respectively. Since the soil parent material in this study is limited to uniform alluvial deposits, the soil weathering index is used here to distinguish the gradient stages of soil development. We have also provided an explanation in the revised manuscript. (Please see L167-168 in the revised version)

Q5: Explain C55 – C85 stages – How were they calculated or determined? This clarification will enable the easy reading.

R: Thank you for your suggestion. C55-C85 represents the chemical index of alteration from 55 to 85. We have provided a detailed explanation and clarification of its calculation formula in section 2.2 of the revised manuscript. (Please see L92-101 in the revised version)

Q6: Figure 4 Give explanation to both axes X and Y

R: Thank you for your suggestion. The X-axis, labeled C55-C85, represents the chemical index of alteration from 55 to 85. The Y-axis represents the content of SOC and SIC. We have revised the figure 4 accordingly. (Please see L182 in the revised version)