Carbon Accumulation and the Possibility of Carbon Losses by Vertical Movement of Dissolved Organic Carbon in Western Siberian Peatlands
Xiaogang Tong
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsSee attached referee report
Comments for author File: 
Comments.pdf
Author Response
Dear reviewer,
thank you for your work. All your reccomendaations were acepted.
Author Response File: Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsI've reviewed the manuscript submitted to Forests titled "Carbon Accumulation and the Possibility of Carbon Losses by Vertical Movement of Dissolved Organic Carbon in West Siberian Peatlands". This research design appears to be well-thought-out and the results are clear and informative. The use of AMS radiocarbon dating to determine the ages of DOC and POC fractions is a valid approach, and the significant findings about age differences between DOC and POC in different layers of peat add to the current body of knowledge. The hypothesis that DOC continuously moves down into the mineral sediment beneath the peat, mixing younger and older carbon, is a logical explanation for the observed dating discrepancy. Moreover, relatively few studies exist on the differences in the ages of DOC and POC at the same soil depths. However, I think the manuscript still needs a major revision in some scientific expression and viewpoint discussion before accepted by the journal.
Abstract
1. Line 16-17: While you provide a numerical estimate of the average rate of DOC downward movement, it would be beneficial to also include an estimated rate of associated carbon loss, as mentioned earlier in the abstract. This could strengthen conclusions and give a more complete picture of the potential implications of findings.
Introduction
1. Line 21-23: “Unlike most other ecosystems …… decomposition”. Please provide more precise sources or references for this argument.
2. Line 26: You could consider adding an introductory sentence at the beginning to smooth the transition from the broader context to the specific topic of Siberian peatlands.
3. Line 43-51: The paragraph about global warming increasing DOC concentrations seems slightly detached from the previous discussion and could be better integrated into the overall argument.
4. Line 45: “…but 0.2-0.7 µm filter pore size are also common” Since your study used 0.45μm filter membrane to filter DOC, this supplementary sentence can be deleted directly.
5. Line 67: the phrase “all of these happen occasionally” seems slightly informal for a scientific context; it might be better to rephrase this to “each of these occurs intermittently”.
6. Line 76-77: the phrase “in the eventuality that differences in the ages of DOC and POC at the same depths were revealed” is somewhat cumbersome and could be simplified. A possible rephrase might be “if differences in the ages of DOC and POC at the same depths are found”.
Materials and Methods
1. Some sentences could benefit from slight restructuring for clarity and ease of reading. For example, “The total area of the Mukhrino peatland is 65 km2. The Mukhrino field station is operated by...” could be merged into one sentence for better flow: “Covering an area of 65 km2, the Mukhrino peatland hosts a field station operated by...”.
2. Line 134-135: The phrase “In case of lower gap occurred the peat sample was cut out by a knife and dating was not provided.” could be clarified for better understanding. Perhaps it could be rephrased as “If a gap occurred in the lower sections of the core, the peat sample was manually extracted using a knife, and no dating was performed on these sections.”
3. Line 139-141: There might be room for improvement in the sentence “These pieces were moved … to avoid contamination.” It could be more clearly written as “Using a clean knife, we transferred these pieces into labeled plastic zip-bags, taking care to minimize contact with the environment to prevent contamination.”
4. L154: The use of the phrase “our own experience” could be perceived as somewhat vague. While this likely refers to the researchers’ expertise in identifying plant remains, it might add clarity to specify if any established guides or references were also used alongside their personal expertise. Similarly, the phrase “key samples data bank” might benefit from additional context. Is this an internally maintained database? Are these samples previously collected and identified by the team or sourced externally? Providing such details would strengthen the description.
5. L170-172: While the list of cores at the end is clear, adding why certain cores were chosen for certain tests would add value to the context and rationale behind the selection.
6. L188: In the phrase “This fraction (> 1.6 < 36 µm) was defined as particulate organic carbon (POC).”, it might be clearer to specify that this refers to particle size, i.e., “This fraction, which has a particle size between 1.6 and 36 µm, was defined as particulate organic carbon (POC).”
7. L219-221: “Bulk density, ash content and carbon content were not all measured in Cores 2, 18, 27 and 31 (see above), so for these cores ACAR was calculated at least partly on the basis of peat types...” - It could be helpful to briefly explain why these measurements were not taken for these cores, as it's not immediately clear from the preceding text.
8. L224: The terms “long-term rate of carbon accumulation (LORCA)” and “long term peat accumulation rate (lPA)” are similar enough that some readers may get confused. Consider differentiating them more explicitly.
9. L244: “Equation 5 calculates how fast the horizontal front of DOC moves downward during the time period under consideration” - This sentence is somewhat confusing. If Equation (5) is measuring vertical movement, why is it referred to as the ‘horizontal front’?
10. L249-L252: “cc = C × v × 104, (6) where cc (mg m−2 yr−1) is the amount of carbon lost per square metre, C (mg cm−3) is the DOC concentration in the deep peatland water, which ranges between 0.06 mg cm−3 [26] and 0.2 mg cm−3 (based on our unpublished estimation)” - It would be better to separate out the information about the DOC concentration range into a separate sentence for clarity.
Results
1. L261-L262: “About two-thirds of the peat deposit is composed of Sphagnum peat with thin interlayers of cottongrass—Sphagnum or sedge—Scheuchzeria—Sphagnum peat types...” - The use of em dashes here makes it somewhat difficult to follow which species are associated with which type of peat. More clarity could be achieved by rephrasing or breaking up the sentence.
2. L263-L266: “The most abundant types are Sphagnum fuscum peat (22.5% of the peat deposit), Sphagnum hollow peat (S. balticum, S. papillosum; 12.0%) and mixed Sphagnum ombrotrophic peat...” - Here, it would help if the author clarified whether the percentages refer to the entire peat deposit or just the Sphagnum peat portion mentioned in the previous sentence.
3.L331-L335: Please provide direct evidence for the claim that “… mineral sediments covering the peatland during an extreme flooding event 1,000–2,000 years ago, or a surface fire.” might have caused high ash content. More concrete evidence is needed to support this hypothesis.
4.L354: How was the downward movement of DOC calculated? What is the relevance of negative values? Providing this information would make the findings more accessible and meaningful.
5.L356-L357: Some certain statements, those about calibration uncertainty affecting uppermost values, could be explained more thoroughly.
Discussion
1. L363: The importance of studying 14C dating of peatlands and greenhouse gas dynamics is mentioned but not explored in depth. Please try to discuss this briefly.
2. L375: One slight confusion is the statement “In [50] showed similar PA values...” - it's unclear who or what “[50]” refers to. Is it a study or author? Additionally, integrating references into the text rather than stating “In [50]” would enhance the flow of the writing.
3. L383- L384: A deeper elaboration on why this research and the majority of published peat age-depth models show a concave shape would be beneficial.
4. L444-L478: It may help to have a brief conclusion that ties together the different possibilities discussed and their implications on the study of peatlands.
5. The language used in discussion is scientific and appropriate for an academic paper. The complex sentences are generally well structured, although there are some grammatical inconsistencies. For example, L458: “In [39] showed a possible path...” should be corrected to “Reference [39] showed a possible path...”. Furthermore, L435: “[54] have previously taken” should be corrected to “[54] has previously taken”; L483: “In [15] used a vertical hydraulic conductivity value...” should be revised to “Reference [15] used a vertical hydraulic conductivity value...”. Some phrases like “the labile phase”, “epigenetic permafrost”, and “piezometric gradient” might need further clarification or definition. And the readability of the text would benefit from addressing these issues.
6. L497-L499: It might be beneficial to elaborate more on why the observed discrepancies in DOC and POC ages matter and how they impact the overall understanding of peatland carbon balance.
Author Response
Dear reviwer, the resond is added in the doc-file.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsDear Authors,
The paper is devoted to explore the behaviour of dissolved organic carbon in a peat profile in Western Siberia and to estimate the apparent rate of DOC downward movement. In my opinion the research study has been well-conduted. I have some comments and recommendations for improving the manuscript and highlighting the innovative approach in your research design.
1) Abstract and introduction
Please highlighting an innovative approach in the abstract and introduction of a research paper which is essential to grab the reader's attention and convey the uniqueness of your study. What is novel in your research study?
2) Results and discussion
What about comparative studies on age-depth models of a peat-bog in Central-Western Europe? Are you able to compare results in table form concerning study area, methods, results etc.? I have found another one research study on peat age-depth models in Poland.
https://doi.org/10.2458/56.16467
3) Discussion
Please add a paragraph on state-the-art machine learning techniques that are powerful and innovative tools for determining features (for example soil carbon stock, carbon uptake, carbon sinks, soil-vegetation biophysical parameters etc.) that might support research studies on peat age-depths. I recommend to mention on XGBoost (https://www.mdpi.com/2072-4292/15/9/2392) and Random Forest (https://www.mdpi.com/2073-445X/12/8/1587) to be adapted at peatlands.
4) Conclusions
You wrote at lines [479-487]: "In our study we estimated an average apparent rate of DOC downward movement of 0.047+-0.019 cm yr-1
There was a slight tendency for the rates to decrease, by 2–10 times, towards the mineral bottom. The most likely reason is low vertical hydraulic conductivity in the deep, dense and well-decomposed basal peat [3]. A limited number of publications estimate the rates of DOC vertical movement. In [15] used a vertical hydraulic conductivity value of 31.5 cm yr−1 to estimate DOC vertical transport in the UK. This value exceeds our results by
∼600 times because the study was based on potential water movement that varies significantly with degree of saturation and due to the physical properties of peat [18]. However, this value might be used as a potential rate of DOC downward movement. It has to be regarded as a maximum possible velocity, i.e., as a limiting factor."
In my opinion it would be better to figure out another study case that could confirm your results at closer level (Is 600 times differentiation level between two comparative studies appropriate?), or provide at the end of the conclusions next steps in the future for next your studies confirming or improving the results.
Kind regards
Author Response
Dear reviwer, the resond is added in the doc-file.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsDear editor:
After revision by the authors, I found that the paper demonstrated a high level of academic rigor in terms of research design, data analysis, interpretation of results, and literature citation. The authors of the paper have addressed the apparent rate of DOC downward movement in depth and provided novel insights, all of which help to move research in the field forward. Below are a few of the main reasons why I support the acceptance of this paper:
The research question is clear and important: The use of AMS radiocarbon dating to determine the ages of DOC and POC fractions is a valid approach, and the significant findings about age differences between DOC and POC in different layers of peat add to the current body of knowledge. The research not only responds to existing scholarly debates but also expands our understanding of the field.
The methodology is sound and adequate: the AMS radiocarbon dating used by the authors is appropriate to the question being explored and is rigorously executed.
The results are convincing: the results presented by the authors are consistent with the research hypotheses and are supported by appropriate statistical analysis. The results section clearly presents the findings of date delay between DOC and POC and the authors properly explain the significance of the findings of in the discussion.
In summary, I consider the paper to be of high academic value and recommend that it be accepted for publication.
Best wishes.
Author Response
Thank you for your review and appreciation of our work.
Reviewer 3 Report
Comments and Suggestions for AuthorsDear Authors,
Thank you for you explanations and clarifications. Please mind my two questions for correcting the structure of the manuscript.
1. No Conclusions section is written. Please add missing section.
2. You wrote in Author's reply:
"Thank you for these interesting publications. We decided to add an additional reference to Random Forest method as more close to our topic."
Indeed, you added one reference in Materials and methods. However we did not understand each other. For making complete Discussion section I asked for adding a paragraph on the new opportunities concerning state-the-art machine learning techniques that are powerful and innovative tools for determining features (for example soil carbon stock, carbon uptake, carbon sinks, soil-vegetation biophysical parameters etc.) that might support research studies on peat age-depths. The reason I suggested citing eXtremege Gradient Boosting from https://www.mdpi.com/2072-4292/15/9/2392 is that XGBoost, renowned for its efficiency and versatility, emerges as a powerful tool for peat age-depth research studies owing to several key strengths. Its adeptness at capturing nuanced non-linear relationships and intricate variable interactions proves invaluable in unraveling the complexities inherent in the relationships between peat age and depth. The algorithm's feature importance scoring facilitates the identification of pivotal variables steering peat formation dynamics. Noteworthy is its resilience to outliers and capability to handle missing data, contributing to the robustness of outcomes. XGBoost's ensemble learning methodology enhances overall model performance, complemented by regularization techniques that curb overfitting. The algorithm's scalability is especially advantageous for managing the voluminous datasets often encountered in peat age-depth investigations. Nevertheless, a judicious application of XGBoost mandates a nuanced understanding of the dataset's nuances and research objectives, ensuring a meticulous alignment of the algorithm's principles with the specific demands of the research domain.
Kindest regards
Author Response
Dear reviwer,
thank you for your work and suggestions. We investigated the suggested algorithm and found it valuable for our future investigation. We added several lines on the topic to the Discussion section (lines 426-433).
The Conclusion section has been added to the manuscript. It summarizes our results and works out future plans (lines 539-557).
