Effects of Forest Conversion on the Stocks and Stoichiometry of Soil Carbon, Nitrogen, and Phosphorus at a County Scale in Subtropical China

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Abstract. The purpose of the work is stated, but there is no scientific analysis of the results. For instance, the designation "carbon limitation (C:N < 8)" is open to question with regard to its suitability for characterizing the organic matter of the humus horizon in the tea gardens. The results are presented without analyzing differences in land use (fertilization, ploughing or harvesting and so on) between the study areas.

1. Introduction. The introduction explains the extent to which the topic has been studied. How C, N and P ratios in soils change under different types of forest conversion and then different agricultural practices not clear. Objectives of the study. First, to quantify carbon (C), nitrogen (N) and phosphorus (P) contents and stocks in the studied soils; second, to compare changes in C, N and P stoichiometry in soils under the influence of different types of natural resource management; third, to identify factors causing changes in C, N and P stocks and ratios in soils.

2. Materials and methods. 2.1. Study area. The study area is described in a rather general manner. For instance, a considerable diversity of soil-forming rocks is indicated. It would have been preferable for plots to have been selected in the area in question so that the soil-forming rocks are identical. The authors further state (2.2. Experimental design and sampling): «plots with similar or identical soil parent material were selected». What exactly were the soil-forming rocks on the sample plots, what kind of soils were formed there, there is no information. The plots were selected according to the prevailing vegetation type and the intended management use. The positioning of key sites of different types in areas with varying elevations (Figure 1) will also may exert an influence on the results.

2.3. Soil testing. Soil analyses were conducted on samples prepared using a method that removed coarse soil (particles greater than 2 mm). However, the results of the analyses were not adjusted for the presence of coarse particles (gravel), nor was the potential impact of this factor on the estimated elemental reserves indicated.

So, the following significant deficiencies have been identified in the conducted research.

1. The content of the examined macronutrients (carbon, nitrogen, phosphorus) is influenced not only by the type of vegetation and its agricultural use or urbanization, but also by variations in soil-forming rocks, soils and the location of key site types (different altitude). 2. The calculation of element stocks (pools) should be confirmed by data on the insignificant amount of coarse-grained mineral particles present in all soils of key sites, as this is the only basis on which element stocks can be calculated with any degree of accuracy. 3. One last point, but not the least. It is insufficient to judge soil processes based solely on elemental ratios. The so-called ‘carbon limitation (C:N < 8)’ may indicate the rapid humification of the fall and the formation of humus compounds, which would provide high potential soil fertility. However, such general assessments should be made with greater caution.

Author Response

1. The purpose of the work is stated, but there is no scientific analysis of the results. For instance, the designation "carbon limitation (C:N < 8)" is open to question with regard to its suitability for characterizing the organic matter of the humus horizon in the tea gardens. The results are presented without analyzing differences in land use (fertilization, ploughing or harvesting and so on) between the study areas.

Response: In the revised manuscript, we added details of important results in Lines19-23: “The soil C:N:P stoichiometric ratios for BF and its converted PF, TG, GS, and CL were sequentially decreased as follows: 444.8:24.2:1, 95.0:10.0:1, 30.2:3.9:1, 23.1:3.7:1, and 19.4:1.9:1, respectively. Within the altitude (AL) span of 180 to 1200 m were surveyed, AL decided the type of forest conversion and significantly influenced the stock levels and stoichiometric ratios of soil C, N, and P”.

Meanwhile, inferences about tea plantations that were not particularly confident have been deleted (Line18: “with carbon limitation”).

In addition, we have explained in section “2.2. Experimental design and sampling” about the overall differences in fertilization, tillage or harvesting etc. between the different types of sample plots in the study area (Lines119-127: “plantation forests (PF), TG, CL, and GS. BF consist of evergreen BF dominated by Castanopsis and Lithocarpus and are unfertilized and unharvested. PF include Chinese fir forests, Masson pine forests, and bamboo forests, all of which receive a small amount of N fertilizer or compound fertilizer and remain largely uncut. TG are dominated by black tea and rock tea plantations, with tea harvested in April–May each year. The CL category includes paddy fields and vegetable fields, whereas the GS category comprises grasslands in parks, along roads, and in residential areas. CL and GS are harvested according to the growing season of different plants. The latter three forest conversion types apply mainly N and P fertilizers or complex fertilizers.”).

We have admitted that the agricultural land in the study area is dominated by non-intensive management and that extensive field surveys have not yet been able to collect specific information on tillage. Therefore, we emphasized in our conclusions the quantification of follow-up studies taking into account habitat and natural resource management (Lines 441-446: “Given the diverse ecological contexts of hilly and mountainous regions and the variability in natural resource management practices, future research should simultaneously quantify factors such as elevation, years of cultivation and their management practices (e.g., fertilization, ploughing, or harvesting) and comprehensively examine soil physicochemical and biological properties to delve into the decoupling mechanisms of soil C, N, and P during forest conversion.).

 

2. Introduction. The introduction explains the extent to which the topic has been studied. How C, N and P ratios in soils change under different types of forest conversion and then different agricultural practices not clear. Objectives of the study. First, to quantify carbon (C), nitrogen (N) and phosphorus (P) contents and stocks in the soils; second, to compare changes in C, N and P stoichiometry in soils under the influence of different types of natural resource management; third, to identify factors causing changes in C, N and P stocks and ratios in soils.

Response: We appreciated your comments and suggestions. We have now made some revisions to the introduction section, including the formulation of the research hypothesis and the revision of the language presentation in the purpose of the study section. We have revised these sentence and now they read：" In this study, we tested two hypotheses: (1) soil C, N, and P stocks and stoichiometric ratios are significantly impacted by forest conversion; and (2) the AL gradient essentially determines the type of forest conversion and affects soil properties as well as the content and ratios of C, N, and P. Therefore, the present study aimed to (1) quantify C, N, and P contents and stocks in the soils from different types of forest conversion; (2) compare changes in C, N, and P stoichiometry in soils under different types of forest conversion; and (3) identify factors (including AL) that affect changes in C, N, and P stocks and ratios in soils (Lines 89-98). "

 

3. Materials and methods. 2.1. Study area. The study area is described in a rather general manner. For instance, a considerable diversity of soil-forming rocks is indicated. It would have been preferable for plots to have been selected in the area in question so that the soil-forming rocks are identical.

Response: In the revised manuscript, we further verified the parent material of the soil in the study area and corrected the original statement (Lines113-115: “ The soil’s parent rocks are predominantly composed of volcanic rock and coarse-grained granite, with the soil being predominantly lateritic (approximately 75%) and generally acidic.”).

 

4. Materials and methods. The authors further state (2.2. Experimental design and sampling): «plots with similar or identical soil parent material were selected». What exactly were the soil-forming rocks on the sample plots, what kind of soils were formed there, there is no information. The plots were selected according to the prevailing vegetation type and the intended management use. The positioning of key sites of different types in areas with varying elevations (Figure 1) will also may exert an influence on the results.

Response: In the section "2.2. Experimental design and sampling", we clarified that within the research altitude range, the rocks of the soil samples are predominantly coarse-grained granite (Lines131-134: “We have selected an AL range from 180 to 1,200 m to ensure that the predominant parent material for soil genesis was coarse-grained granite, with the soil types primarily consisting of yellow-red soil (700–1,200 m) and lateritic soil (< 700 m).”).

We recognized your query about elevation and the positioning of different types of locations. The sample plots were primarily laid out along an elevation gradient, as this corresponded to a representative representation of the dominant vegetation types and intended management uses surveyed on site. Although this may still influence the results of the study, the general trend of the findings was somewhat convincing with the assurance of the larger amount of data generated from the centrally distributed sample plots. In addition, our later studies will further reflect on and refine the impact of these uncertainties.

 

5. Materials and methods. 2.3. Soil testing. Soil analyses were conducted on samples prepared using a method that removed coarse soil (particles greater than 2 mm). However, the results of the analyses were not adjusted for the presence of coarse particles (gravel), nor was the potential impact of this factor on the estimated elemental reserves indicated.

Response: Since the studied sample plots were dominated by yellow-red soil (700-1200 m) and lateritic soil (< 700 m) with a small percentage of gravel, we can ignore the influence of this aspect on the calculation of carbon, nitrogen and phosphorus stocks. Similar treatments and calculations are based on corresponding references (cited in [1]). In addition, in the section "2.2. Experimental design and sampling", we added “Then, the samples were sifted through a 20‑mm mesh (gravel samples on the sieve were minimal and could be ignored [1]) for soil particle size analysis)”(Lines 153-155).

 

6. So, the following significant deficiencies have been identified in the conducted research.

1. The content of the examined macronutrients (carbon, nitrogen, phosphorus) is influenced not only by the type of vegetation and its agricultural use or urbanization, but also by variations in soil-forming rocks, soils and the location of key site types (different altitude).

Response: We agreed with your points, but we chose a relatively small scale county-wide study. Meanwhile this study focused on the effects of major forest conversion types on soil carbon, nitrogen and phosphorus content. The coefficients of variation for soil C, N, P content and stocks for the same site type were small in our analysis, and more differences were found in different forest conversion types (Figs. 2, 3). Therefore, our results confirm that forest conversion types significantly affect soil carbon, nitrogen, and phosphorus content, storage, and stoichiometric characteristics.

 

7. The calculation of element stocks (pools) should be confirmed by data on the insignificant amount of coarse-grained mineral particles present in all soils of key sites, as this is the only basis on which element stocks can be calculated with any degree of accuracy.

Response: We believed that there is much merit in the analysis. However, the methods for calculating C, N, P stocks in soils that are not equally situated in this manuscript are widely recognized and applied (e.g., citing literature [1,2,10,28]). Using the existing calculation methods, we have derived elemental stocks with a clear attribution to forest conversion types. This suggests that the statistics of the various sites corresponding to the forest conversion type can represent the overall situation of the counterpart. Therefore, we feel that data processing of existing manuscripts is still feasible.

 

8. One last point, but not the least. It is insufficient to judge soil processes based solely on elemental ratios. The so-called ‘carbon limitation (C:N < 8)’ may indicate the rapid humification of the fall and the formation of humus compounds, which would provide high potential soil fertility. However, such general assessments should be made with greater caution.

Response: We were sorry to admit that the revised manuscript did not fully take into account the possible bias of C/N/P ratios of different site types in judging soil status when applying the results of the existing research. Therefore, the current manuscript has deleted the inference of TG soil with “carbon limitation” from the abstract.

Reviewer 2 Report

Comments and Suggestions for Authors

Research is relevant and interesting.

 

1. No research hypothesis is presented in the introduction;

2. In the chapter „Materials and Metods“ the used statistical analysis methods must be described in more detail;

 

3. In the figures No. 2 and 3 it is not explained what the vertical bars indicated;

4. I recommend to provide more detailed conclusions with a proposal for the future;

5. I recommend not to use old literature sources.

Author Response

1. No research hypothesis is presented in the introduction;

Response: Thank you very much for your comments. Two hypotheses in Introduction (Lines 89-92: “In this study, we tested two hypotheses: (1) soil C, N, and P stocks and stoichiometric ratios are significantly impacted by forest conversion; and (2) the AL gradient essentially determines the type of forest conversion and affects soil properties as well as the content and ratios of C, N, and P. ”) have now been added to the manuscript and tested in the discussion.

 

2. In the chapter “Materials and Metods” the used statistical analysis methods must be described in more detail;

Response: The revised manuscripts have been supplemented with further details of the relevant content (Lines176-195: “The digital AL data was taken from Google earth (resolution, 4 m), and the map was generated using Geographic Information System software. Based on the latitude and longitude information of each plot, a schematic diagram of the soil sampling points was created using ArcMap 10.8 software (Esri, Redlands, CA, USA). The primary data were analyzed without any transformation. Microsoft Excel was used to conduct a descriptive statistical analysis of the data. SPSS 27 software (SPSS. Inc., Chicago, IL, USA) was employed to perform univariate analysis of variance (ANOVA) (P < 0.05) for soil C, N, and P contents, stocks, and stoichiometric ratios. Correlation heatmaps between the basic physicochemical properties of the soil were constructed using GraphPad Prism 5 (GraphPad Software. Inc., La Jolla, CA, USA) and Origin 2021 (Origin Lab. Inc., Massachusetts, USA). Redundancy analysis (RDA) was conducted using the Canoco 5 to identify the key factors influencing soil C, N, and P stocks and stoichiometric ratios. During RDA, explanatory factors exhibiting clear collinearity trends were removed to ensure the accuracy of the model operation. Logarithmic normalization was also performed on the variables to mitigate the impact of dimensional differences on the results, facilitating a better measure of the contribution of explanatory variables to the constraint axis through the absolute value of the canonical coefficient (the regression coefficient of the model”).

 

3. In the figures No. 2 and 3 it is not explained what the vertical bars indicated;

Response: Thank you very much for your comments, we have added an explanation of the representative indicators of the bar chart in the figures No. 2 and 3 (Lines 228-230 and 241-243: “Bars represent the mean and standard error. Different lowercase letters in the same significant difference condition indicate significant difference among forest conversion types at P < 0.05.”).

 

4. I recommend to provide more detailed conclusions with a proposal for the future;

Response: Your comments are greatly appreciated. The shortcomings of this study are further explored in the revised manuscript and corresponding recommendations are made in “5. Conclusions” (Lines 441-446: “Given the diverse ecological contexts of hilly and mountainous regions and the variability in natural resource management practices, future research should simultaneously quantify factors such as elevation, years of cultivation and their management practices (e.g., fertilization, ploughing, or harvesting) and comprehensively examine soil physicochemical and biological properties to delve into the decoupling mechanisms of soil C, N, and P during forest conversion.”).

 

5. I recommend not to use old literature sources.

Response: In the revised manuscript, we removed duplicate citations ([24],[46]) replaced some older, not particularly important references ([3], [5], [11], [17]) with newer, relevant ones, and added two new ones ([45], [46]) as needed to revise the manuscript.

 

 

Reviewer 3 Report

Comments and Suggestions for Authors

1.      The abstract is adequate. However, more significant data should be presented. For instance, the increases and reductions of C, N, and P contents. Moreover, “Altitude” has not been presented in the methodology and has not been clearly interpreted in the results of the abstract either.

2.      “Liu et al. (2022)” is not a correct citation. This also happens in the discussion.

3.      The “Wuyishan City” and “China” paragraphs should be merged and summarized.

4.      These “The city possesses one of the most pristine, extensive, and typical subtropical native forest ecosystems in the world at its latitude” and “It is also the birthplace of the world’s black and oolong teas” is not relevant.

5.      How was the “An area of 20 × 20 m was marked out in each plot” chosen?

6.      The specific altitude should be presented for each site.

7.      The results are well presented.

8.      This “Consequently, the soil C and N status in southern China is predominantly determined by the type of forest conversion” should be cited. Line 264-265

9.      This is well discussion, but the location of data source should be cited, e.g line 271, 289

10.  The conclusion should remove the second sentence, for two highlight findings, they should make shorten by rules.

Comments on the Quality of English Language

Minor editing of English language required

Author Response

1. The abstract is adequate. However, more significant data should be presented. For instance, the increases and reductions of C, N, and P contents. Moreover, “Altitude” has not been presented in the methodology and has not been clearly interpreted in the results of the abstract either.

Response: In the revised manuscript, we have added details of important results in Lines 19-23: “The soil C:N:P stoichiometric ratios for BF and its converted PF, TG, GS, and CL were sequentially decreased as follows: 444.8:24.2:1, 95.0:10.0:1, 30.2:3.9:1, 23.1:3.7:1, and 19.4:1.9:1, respectively. Within the altitude (AL) span of 180 to 1200 m surveyed, AL decided the type of forest conversion and significantly influenced the stock levels and stoichiometric ratios of soil C, N, and P. ”.

In addition, we supplemented the scope of “altitude” in the Abstract (“Within the altitude (AL) span of 180 to 1200 m surveyed, AL decided the type of forest conversion and significantly influenced the stock levels and stoichiometric ratios of soil C, N, and P.”). We have also added the appropriate information in Exhibit 1 for the latitude, longitude, and elevation of the sample plots in “Table S1” (Table S1. Geographical coordinates information and altitude distribution for sampling sites of various forest conversion types.) of the revised manuscript supplementary.

 

2. “Liu et al. (2022)” is not a correct citation. This also happens in the discussion.

Response: We appreciated your comments. We have revised a number of similar references in the original manuscript, as detailed in the existing manuscript (Line 56 and 289).

 

3. The “Wuyishan City” and “China” paragraphs should be merged and summarized.

Response: We agreed with your suggestion! We have merged these two passages, as detailed in Lines 66-88 of the revised manuscript.

 

4. These “The city possesses one of the most pristine, extensive, and typical subtropical native forest ecosystems in the world at its latitude” and “It is also the birthplace of the world’s black and oolong teas” is not relevant.

Response: We have deleted the content of the corresponding sentence in the revised manuscript.

 

5. How was the “An area of 20 × 20 m was marked out in each plot” chosen?

Response: We selected plots with homogenous and typical vegetation to define our sample plots through a site survey. We have further added relevant details in “2.2. Experimental design and sampling” in the revised manuscript (Lines131-144).

 

6. The specific altitude should be presented for each site.

Response: Figure 1 in the text showed the digital elevation map of the study area, which showed the latitude, longitude and elevation information. Also, we have added the appropriate information in “Table S1” (Table S1. Geographical coordinates information and altitude distribution for sampling sites of various forest conversion types.) of the revised manuscript supplementary.

 

7. The results are well presented.

Response: We very much appreciate your comments.

 

 

8. This “Consequently, the soil C and N status in southern China is predominantly determined by the type of forest conversion” should be cited. Line 264-265

Response: We have added the corresponding literature citation (Line 301: [1]) in the revised manuscript.

 

9. This is well discussion, but the location of data source should be cited, e.g line 271, 289

Response: In the revised manuscript, we have added the location of data from the cited literature (Lines 288-301, 335, 353-354).

 

10. The conclusion should remove the second sentence, for two highlight findings, they should make shorten by rules.

Response: We have removed the second sentence in the conclusion. Meanwhile, we have modified the presentation of our conclusions and made some desirable suggestions for future research (Lines 427-436).

 

11. Comments on the Quality of English Language: Minor editing of English language required

Response: Your comments are greatly appreciated. The revised manuscript has been embellished in English.

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors provided a comprehensive response to the reviewer's comments, implementing the necessary corrections to the manuscript.

However, one question remains regarding the section entitled "Authors' Responses to Reviewer's Comments."

IT WOULD NOT APPEAR THAT THE INFORMATION PRESENTED IN THE POINTS 3 AND 4 IS ENTIRELY IDENTICAL. IT SEEMS PLAUSIBLE TO SUGGEST THAT THE AUTHORS MAY HAVE BEEN SOMEWHAT HASTY IN THEIR APPROACH AND DID NOT VERIFY THE INFORMATION PERTAINING TO SOILS AND SOIL-FORMING ROCKS. (The details, please, see below)

Authors' Responses to Reviewer's Comments

Author's Notes

Point 3. Materials and methods.

Response.

The soil’s parent rocks are predominantly composed of volcanic rock and coarse-grained granite, with the soil being predominantly lateritic (approximately 75%)…

Point 4. Materials and methods.

Response.

… the predominant parent material for soil genesis was coarse-grained granite, with the soil types primarily consisting of yellow-red soil (700–1,200 m) and lateritic soil (< 700 m)

 

About the edited manuscript.

 

Lines 16-17. “The results showed that the mineralization of soil C and N was pronounced (C:N<20), especially in TG soil”.

Sorry, I am unable to ascertain the meaning of the sentence. It would be preferable to remove that sentence.

Lines 52-53. “surface soil”

Please elucidate the meaning of the term in question. Additionally, specify the soil thickness for which the stocks, contents and stoichiometry of C, N, P have been calculated. Is this the depth of the arable horizon, which may be 20 or 30 cm? Alternatively, is it even more?  50 cm for example. Please provide an explanation of this term in the manuscript as a whole. Not only for the lines 52-53.

Line 120. “We have selected an AL range r from 180 to 1,200 m to ensure…”

r – is it misprint?

Lines 121-122. See please the first question.

Line 126. “soil sampling at a depth of 0–20 cm” – So, You estimated the stocks the the 20 cm depth. Is the same in the studied literature (lines 52-53 and so on)?

Line 140. “gravel” What is the size of the gravel? Particles more than 2 mm?

Line 142. “sifted through a 20-mm mesh” Is it? 2 mm or 20 mm?

Lines 203-204. Why are stock values given in mg/ha in the text and Mg/ha in Figure 3? I suggest, there is an error in the text of the manuscript. It is necessary to put M large. It's important: mg stands for milligrams.

Lines 266-267. It would be interesting for the reader to know what kind of quantities were considered to be high in the literary source. If you give information from the literature, why not give quantitative estimates of the quantities described? And so on for the all manuscript.

Lines 311-312. “The average value of soil C:N ratio was below 20 (Fig. 3), indicating substantial

mineralization of soil C and N”

Lines 311-312. Mineralisation is not the only process by which litter is transformed in the soil. Mineralisation is accompanied by the synthesis of complex organic molecules that are stable outside the living organism. This process is called humification. Humus substances contain more nitrogen than fresh plant litter. The authors of the manuscript appear to have a different scientific stance. Does the process of mineralisation only occur in plant debris in the soil?

Author Response

Please refer to the attachment. Thanks.

Author Response File: Author Response.pdf