I have read “Diachronic assessment of soil organic C and N dynamics under long-term no-till cropping systems in the tropical upland of Cambodia.” The manuscript describes a study where soil organic carbon (SOC) mass fractions (“concentrations”), stocks, and particulate/mineral pools were measured at a ten-year interval across different crop types (maize, soybean, and casava) and management practices (monocrop with conventional till without cover crop, monocrop with no till with cover crop, and two phases of crop rotations with no till and cover crop). The authors report that SOC concentration, SOC stocks, and SOC pools generally increased near the surface under no-till treatments, but generally showed fewer changes at depth. N concentrations and stocks generally increased near the surface but decreased at depth under no-till. Space-for-time sampling (“synchronic”) generally underestimated SOC stock changes relative to longitudinal sampling (“diachronic”). The authors conclude that no-till cover crop systems may help to increase SOC, but more work is needed to understand N dynamics.

The dataset is comprehensive, the topic is of general interest, and manuscript is generally well-written. However, I have some suggestions for improvement.

General comments:

The manuscript is too long, primarily because of the details provided in the results text. The figures and tables contain the details that the reader can reference, and the text should serve to bring attention to the important findings. Therefore, it is not necessary to repeat in the text every result shown in the figures and tables. I would also recommend combining the maize, soybean, and casava results into one section (for both the results and the discussion), first pointing out the similarities in the results (there are many), then pointing out the differences. This would not only reduce the amount of text but also improve the flow.

Related to the last point, the authors could reduce the number of tables and figures in the main text by focusing on specific questions. Right now, there are five tables and six figures, and many of the figures are multipaneled. Tables 2 and 5 seem like they could easily be moved to the supplemental. If the manuscript’s focus is “storage,” “stocks,” and “accumulation,” as indicated in the first three sentences of the abstract, then it also seems like it is unnecessary to report the C and N concentrations in the main text (i.e., Figures 1 and 2 could be moved to supplemental).  

Under each crop type (maize, soybean, and casava), the conventional till system does not have a cover crop whereas the no-till systems have cover crops. Therefore, the effects of no-till and cover crops are confounded. This should be corrected throughout the manuscript when referring to “no-till” effects, including in the title.

I found the descriptions of the experiments and the treatments to be insufficient. I had to reference several of the previous publications from this study to understand that 1) the maize, soybean, and casava treatments were “separate” experiments at the same site and, 2) NT2 and NT3 are the two phases of a rotation, and 3) the site has been in various agricultural uses since 1937. These are important details that must be explained sufficiently in the text. There are still some questions that I have about the experimental design that I list in the specific comments below.

The authors use the equivalent soil mass (ESM) approach to calculate changes in SOC and N stocks, which is robust assuming that there are no losses or gains of mineral soil. However, the authors also mention that erosion is an issue in this area, which would cause ESM (and the fixed-depth approach) to be inaccurate. The authors should address this issue, either by providing assurance that significant erosion does not occur at the site, or providing an approximation of erosion rates (and thus the uncertainty of stock change estimates). Also, the C and N concentrations and POM/MAOM values were not calculated using ESM, but these could also be affected by changes in bulk density. I recommend using ESM for these values in addition to the stocks.

Specific comments:

L35-38: The results given at the end of the abstract indicate that the NT system contained cover crops, but that is not mentioned here. A better description of the systems is necessary.

L40-41: It is not necessary to list all the depth increments here, as they are not reported in the abstract.

L48-50, and throughout the manuscript: If the results are not significant, then do not calculate the percent change.

L61: This implies that diversity was explicitly tested in this experiment, which it was not. If diversity is of interest, then it should be tested in the analysis.

L76, elsewhere: The word “inappropriate” could be changed to something more specific like “intensive.”

L81-82: Please provide a citation.

L96: The fate of eroded SOC is difficult to assess. See Sanderman and Chappell 2013 (doi:10.1111/gcb.12030).

L99-100: Please provide a citation.

L101: The term “SOC content” is ambiguous, as it is sometimes used to describe stocks and sometimes used to describe mass fractions (“concentration”). Please be explicit.

L112: NT is part of CA, should “NT” can be removed from this sentence.

L121-122: The introduction should have more information about this. Some of this could be moved from the discussion, e.g., L793-796.

L153-168: This seems out of place. It should come earlier in the discussion because it is part of the “big picture.”

L171-173: What is the hypothesis regarding the POM and MAOM?

L173-174: This isn’t so much a hypothesis as it is a test bias.

L180: What is the slope of the site?

L201-207: The treatment abbreviations for the systems (especially the NT systems) could be improved to better convey similarities and differences. For example, CTM (conventional till monoculture), NTM (no-till monoculture), NTR1 (no-till rotation phase 1), and NTR2 (no-till rotation phase 2).

L205-207: This makes it sound like the NT2-Sb, NT3-Sb, NT2-Cs, and NT3-Cs do not have cover crops, but I don’t believe that is correct.

L221-222: Why weren’t the root inputs estimated, for example, by using a root-to-shoot ratio?

L253: Pyrophosphate is a more common term; otherwise thermophosphate as one word.

Table 1: While this table is very comprehensive, it is very difficult to interpret because there are so many different types of crops. I would recommend making it easier for the reader to understand by putting the crops into categories. For example, main crop could be bolded, cash crop could be underlined, and cover crop could be italicized.

Table 2: It seems like this can be supplemental information that is referenced in the text where appropriate.

L282: A 20-year-old coffee plantation with otherwise identical history to the experimental treatments does not seem like appropriate “reference vegetation.” Reference vegetation would be an old growth forest (e.g., 100 years old) or remnant forest (never cultivated). Readers might assume that the “reference vegetation” contains the maximum SOC stock possible, which is highly unlikely in this case. Moreover, the SOC stock in the “reference vegetation” was only measured to 20 cm deep. I suggest that the reference vegetation data and Figure 3 are removed.

L286: Can the authors explain the rationale for using a different soil sampling strategy in 2021 compared to 2011?

L289: What is the model of the “soil column cylinder auger?”

L297, 301: It appears that SOC and N was measured differently between 2011 and 2021 (different lab, different instrument). Did the authors run any of the samples on both instruments/labs to check for consistency. Or could this difference explain the unexpected decrease in N reported in 2021? For example, a small change in the N calibration between instruments could result in apparent N changes through time.

L351: “computed” could be “conducted.”

L367-371, 512-517: This preamble is unnecessary.

Figure 1. Figure 2: In general, error bars are necessary for graphs. Also, the description does not mention that text in bold brackets in the 2011 panels (presumably) represents a significant decrease.

Table 3, Table 4: Measurements should have units for error (e.g., standard error).

L599: The discussion would be stronger if it started with the takeaway messages and significance of the study.

L619-630: These are detailed results that should not be presented in the discussion.

L665-679: This would fit better with the discussion of POM and MAOM.

L680: This seems like a separate idea from the prior part of the paragraph

L707-711: This should be in the results section.

L714: A major reason why the synchronic method produced lower SOC stock changes than the diachronic results is because the control treatment had increased SOC stocks over time (although it may not have been statistically significant, it was still numerically higher, which affected the calculation).

Figure 6: Statistically significant changes (i.e., different from zero) should be noted, for example with an asterisk. Also, it would be helpful to draw a horizonal line on each graph to represent zero.

L755-764: Could N uptake and/or N priming from the cover crops have resulted in N loss?

L765-791: This section doesn’t add much in addition to the previous section, especially since the comparison the “RV” was only done to 20 cm and the RV vegetation is not likely representative of the pre-agricultural SOC stocks. I recommend removing this.

L796-799: This level of detail should be confined to the results.

L825-839: This is too much detail for the conclusions. Provide a high-level overview of the results and implications.

L846: In addition to “question about N dynamics,” it also raises questions about the sustainability of these systems in general. If the systems are continually losing N, they may not be sustainable. Moreover, SOC has a fixed ranged of C:N, so at some point, C will no longer accumulate if N is being lost.

General comments:

This study explored the soil organic C and total N dynamics in the soil profile of 0-100 cm under long-term no-till cropping systems (conventional tillage, NT monocropping, and NT crop rotation systems) in the tropical upland of Cambodia using a diachronic and equivalent soil mass approach. The results of this study showed that adopting NT cropping systems with diverse crop and cover crop species increased SOC accumulation (SOC concentration, SOC stocks) in the whole depth, by increasing both the C pools in the POM and MAOM size fractions but raised questions about soil N dynamics (TN concentration, TN stocks). However, there are still some important issues about study objectives, result analysis and discussion that need to be addressed before being accepted. Detailed comments are as below.

1.This study mainly aimed to quantify the impacts of CT and different NT cropping systems on SOC and TN dynamics, thus the comparison between diachronic approach and synchronic approach should not be included in the scientific hypotheses. Note that the C pools in the POM and MAOM size fractions should be included in the hypotheses. Suggest moving the results of method comparison to supplementary materials.

2.Multiple crop species were included in these NT cropping systems, which will significantly affect the accumulation of soil organic carbon and total nitrogen. However, the potential mechanisms were not discussed or explained in this study.

3.What are the advantages and disadvantages of the diachronic and synchronic approaches? The background should be described in the Introduction, as this study emphasizes that the diachronic method can prevent biased estimation of SOC accumulation compared to the synchronous method.

4.Unfortunately, this paper reads more like a research report than a scientific article. It is confusing to understand the important results when SOC and total N (TN) are described together without a clear logical order. New subheadings are recommended to distinguish the results for SOC and TN. In addition, it would be better to combine the results for maize, soya and cassava cropping systems into same section rather than using the subheading for each system.

5.Rather than detailing the results of published papers (e.g. L602-630, 631-640, etc.) and replicating the results of this study (e.g. L623-630, etc.), it is more important to effectively discuss the mechanisms by which different crop rotation systems under NT affect SOC and TN in this study.

 Special comments line by line:

Line 31-66. The abstract is generally limited to 400 words or less, please condense the text.

Line 42-58. The results of the SOC and TN dynamic changes under CT should be described in the abstract.

Line 42-58. Since the main results showed the differences under diverse cropping systems during the 10-year period (2011-2021), a clear comparative year (2011 and 2021) should be added at the beginning of these sentences.

Line 42-44. P value > 0.05? if the significant difference was observed, the P value was less than 0.05. Please check and revise.

Line 81-98, 153-168. Suggest integrating these two paragraphs.

Line 170-174. Add the abbreviations (TN) for total N stocks, please check and revise it in the main text.

Line 183. Please check and add the SOC content or SOC stocks.

Line 190. In section 2.2, the planting date and harvest date of multiple crops should be added under different cropping systems.

Line 203-207. Please separately describe the treatment iii and iv.

Line 296-337. Add the information and equations for TN.

Line 389-396, 409-413, 428-431, 444-447, 475-478, 497-498. It is recommended to move the results of the RV period to the supplementary materials.

Line 414-417, 448-452, 499-502. These paragraphs are very unclear, it is recommended to integrate above text with Line402 - 408, Line441 - 443 and Line490 - 496 respectively.

Line707-719. These sentences and Figure 6 should be moved to the materials and methods to compare the difference between these two approaches or moved to the supplementary materials.

Table and Figure.

Please add standard error for each table and figure.

Table 1. The ‘M’ should be as ‘Mz’ in the NT1-Mz, please check and revise.

Table 3, 4 and 5. It is confusing to understand the difference between uppercase letters (diachronic) and lowercase letters (synchronic) in these tables. It is recommended to keep the results of only one main method (i.e., diachronic), and to include the results of another measurement method (synchronic) in the supplementary materials.

Figures. Please add the note in the figure caption to indicate the details of (A), (B), (C), …(E), (F), respectively.

Fig. 3. Please add the note to indicate the details of numbers (i.e., 41.6 and 4.32) in this figure.