the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Nov 2023
Differential vulnerability of mineral-associated and particulate soil organic carbon to nitrogen addition in a subtropical forest
Abstract. Nitrogen (N) deposition rates of terrestrial ecosystems have gradually declined but are still high in some areas, and previous studies have reported that N addition elicits diverse impacts on soil organic carbon (SOC) pools. SOC can be divided into different functional fractions, namely particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). The responses of these fractions to N addition should be elucidated to better understand the changes in SOC pools. We investigated the effects of N addition treatments (+0, +40, and +80 kg N ha−1 yr−1) in a subtropical Castanopsis fabri forest to simulate N deposition. The surface (0–10 cm) SOC content in different fractions, aboveground litter biomass, fine root (diameter < 2 mm) biomass, soil exchangeable cation content, and soil enzyme activity under different N addition treatments were measured. The results showed that 1) N addition exerted a positive effect on POC content but did not significantly affect MAOC content. 2) POC content was negatively correlated with pH and soil enzyme activity and positively correlated with aboveground litter biomass, suggesting that POC accumulation was influenced by aboveground litter input and microbial decomposition. 3) Root biomass was unaffected significantly by the addition of N, which could be responsible for the limited response of MAOC to N addition. Furthermore, a close negative relationship was observed between exchangeable Al3+ and Ca2+ or K+ contents, indicating the presence of a trade-off between negative effects of exchangeable cations on SOC bridging and their positive effects on SOC adsorption, thus resulting in an insignificant reaction of MAOC to N addition. Overall, N addition reduces the persistence and increases the nutrient density of SOC, and MAOC with more protection is less vulnerable to N addition than POC with less protection. By incorporating these nuances into ecosystem models, it is possible to more accurately predict SOC dynamics in response to global change.
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CC1: 'Comment on egusphere-2023-2460', Yalin Hu, 20 Nov 2023
The authors in this paper reported the effects of nitrogen addition on soil particulate (POC) and mineral-associated organic carbon (MAOC) content after a short time in one subtropical forest. The results showed that POC content increased by N addition, but there was no change of MAOC content. Furthermore, the authors stated the increases of aboveground litter production and lowered soil enzymatic activities mainly explained the POC content, and the trade-off of exchangeable cations resulted in no changes of MAOC content. This study can provide us some information on the influences of N deposition on SOC. However, there are many previous studies that have been reported the effects of N addition on soil C pools and the chemical exchangeable cations. Thus it seemed that novelty of this study is slightly weak, especially considering that only one Castanopsis fabri forest stand and it is not the most typical vegetation in the studied sub-tropic region. More detail minor comments were as follows
- Line 42-43, this sentence is not clear and not robust conclusion. Based on the data of this study, you can not conclude N addition reduced the persistence of SOC and the less vulnerable MAOC because of the less change of MAOC content. It is also not very clear and difficult to understand “the nutrient density of SOC”
- Study site Section, you need to give more detail description of forest stand, such as natural forest or plantation forest, stand age, tree density, and so on.
- Experimental design, it is not robust experiment considering just one time of N addition after 1.5 year when you collected soil sample in December 2021. Moreover, the author determined BG and CBH enzymatic activities just related to cellulose decomposition.
- Result section, SOC content and soil pH values should be necessary and important data and it is better to give these data in text.
- Line 285-286, it is too speculated that the less root biomass explained the lack of MAOC content.
- Line 318-320, it is not reasonable to conclude N addition reduced the persistence of SOC based on the lower MAOC:POC ratio.
- 4, change “aboveground litter biomass” to “ aboveground litter product”
Citation: https://doi.org/10.5194/egusphere-2023-2460-CC1 -
AC1: 'Reply on CC1', Qiufang Zhang, 07 Dec 2023
The authors in this paper reported the effects of nitrogen addition on soil particulate (POC) and mineral-associated organic carbon (MAOC) content after a short time in one subtropical forest. The results showed that POC content increased by N addition, but there was no change of MAOC content. Furthermore, the authors stated the increases of aboveground litter production and lowered soil enzymatic activities mainly explained the POC content, and the trade-off of exchangeable cations resulted in no changes of MAOC content. This study can provide us some information on the influences of N deposition on SOC. However, there are many previous studies that have been reported the effects of N addition on soil C pools and the chemical exchangeable cations. Thus it seemed that novelty of this study is slightly weak, especially considering that only one Castanopsis fabri forest stand and it is not the most typical vegetation in the studied sub-tropic region. More detail minor comments were as follows.
Response: Thank you very much for your kind work and constructive comments, which greatly improved the quality of the manuscript.
Line 42-43, this sentence is not clear and not robust conclusion. Based on the data of this study, you can not conclude N addition reduced the persistence of SOC and the less vulnerable MAOC because of the less change of MAOC content. It is also not very clear and difficult to understand “the nutrient density of SOC”.
Response: We have revised the sentence as “the accumulation of SOC under N addition is primarily driven by POC, and POC with less protection is more vulnerable to N addition than MAOC with more protection”. Please see Lines 40–42.
Study site Section, you need to give more detail description of forest stand, such as natural forest or plantation forest, stand age, tree density, and so on.
Response: This is a natural forest. Since the establishment of the national nature reserve, this forest has not been disturbed by human activities. At the beginning of the experiment, the tree height was 15-20 meters, the diameter at breast height was 20-40 centimeters, and the closure was about 75%. We have added those detail description of forest in the “Study site” section. Please see Lines 114–122.
Experimental design, it is not robust experiment considering just one time of N addition after 1.5 year when you collected soil sample in December 2021. Moreover, the author determined BG and CBH enzymatic activities just related to cellulose decomposition.
Response: We agree with your point of view. We will also focus on the effects of longer periods of N addition in later experiments. BG and CBH are both cellulase enzymes that act on the ends of reductive and non-reductive cellulose polysaccharide chains to release glucose or cellobiose. BG hydrolyzes cellobiose to produce two molecules of glucose. We originally considered enzymes such as polyphenol oxidase and peroxidase for degrading polyphenolic compounds, but short-term N addition had no significant effect on these two enzymes. This may be because short-term N addition did not significantly promote the accumulation of recalcitrant organic carbon in the soil, so it was not included in this study.
Result section, SOC content and soil pH values should be necessary and important data and it is better to give these data in text.
Response: Done. The data is presented in Table 1. Compared with the control, both low-N addition and high-N addition significantly reduced soil pH but significantly increased SOC content. Please see Lines 216–217.
Line 285-286, it is too speculated that the less root biomass explained the lack of MAOC content.
Response: Thank you for your kind suggestion. We have changed the sentence to “fine root biomass was not influenced by short-term N addition in this study, and there was no significant relationship between fine root biomass and MAOM. Therefore, the regulatory role of fine root biomass on MAOM needs further exploration”. Please see Lines 294–296.
Line 318-320, it is not reasonable to conclude N addition reduced the persistence of SOC based on the lower MAOC:POC ratio.
Response: We agree with your point of view and have deleted it.
Change “aboveground litter biomass” to “aboveground litter product”.
Response: Done. We have changed “aboveground litter biomass” to “aboveground litter product”.
Citation: https://doi.org/10.5194/egusphere-2023-2460-AC1 -
AC5: 'Reply on CC1', Qiufang Zhang, 05 Jan 2024
The authors in this paper reported the effects of nitrogen addition on soil particulate (POC) and mineral-associated organic carbon (MAOC) content after a short time in one subtropical forest. The results showed that POC content increased by N addition, but there was no change of MAOC content. Furthermore, the authors stated the increases of aboveground litter production and lowered soil enzymatic activities mainly explained the POC content, and the trade-off of exchangeable cations resulted in no changes of MAOC content. This study can provide us some information on the influences of N deposition on SOC. However, there are many previous studies that have been reported the effects of N addition on soil C pools and the chemical exchangeable cations. Thus it seemed that novelty of this study is slightly weak, especially considering that only one Castanopsis fabri forest stand and it is not the most typical vegetation in the studied sub-tropic region. More detail minor comments were as follows.
Response: Thank you very much for your kind work and constructive comments, which greatly improved the quality of the manuscript. Based on the comments of other reviewers, we have made revisions to the entire text. In this new response letter, we have revised the line numbers.
Line 42-43, this sentence is not clear and not robust conclusion. Based on the data of this study, you can not conclude N addition reduced the persistence of SOC and the less vulnerable MAOC because of the less change of MAOC content. It is also not very clear and difficult to understand “the nutrient density of SOC”.
Response: We have revised the sentence as “the accumulation of SOC under short-term N addition is primarily driven by POC, and the response of different SOC functional fractions to N addition is inconsistent”. Please see Lines 39–41.
Study site Section, you need to give more detail description of forest stand, such as natural forest or plantation forest, stand age, tree density, and so on.
Response: This is Castanopsis fabri natural forest. Since the establishment of the national nature reserve, this forest has not been disturbed by human activities. At the beginning of the experiment, the tree height was 15-20 meters, the diameter at breast height was 20-40 centimeters, and the closure was about 75%. We have added those detail description of forest in the “Study site” section. Please see Lines 110–118.
Experimental design, it is not robust experiment considering just one time of N addition after 1.5 year when you collected soil sample in December 2021. Moreover, the author determined BG and CBH enzymatic activities just related to cellulose decomposition.
Response: We agree with your opinion. We will focus on the effects of longer periods of N addition in later experiments. BG and CBH are both cellulase enzymes that act on the ends of reductive and non-reductive cellulose polysaccharide chains to release glucose or cellobiose. BG hydrolyzes cellobiose to produce two molecules of glucose. We originally considered enzymes such as polyphenol oxidase and peroxidase for degrading polyphenolic compounds, but short-term N addition had no significant effect on these two enzymes. This may be because short-term N addition did not significantly promote the accumulation of recalcitrant organic carbon in the soil, so it was not included in this study.
Result section, SOC content and soil pH values should be necessary and important data and it is better to give these data in text.
Response: Done. The data is presented in Table 1. Compared with the control, both low-N addition and high-N addition significantly reduced soil pH and significantly increased SOC content. Please see Lines 215–216.
Line 285-286, it is too speculated that the less root biomass explained the lack of MAOC content.
Response: Thank you for your kind suggestion. We have changed the sentence to “fine root biomass was not influenced by short-term N addition in this study, and there was no significant relationship between fine root biomass and MAOC. The regulatory role of fine root biomass on MAOC needs further exploration”. Please see Lines 316–319.
Line 318-320, it is not reasonable to conclude N addition reduced the persistence of SOC based on the lower MAOC:POC ratio.
Response: We agree with your opinion and have deleted it.
Change “aboveground litter biomass” to “aboveground litter product”.
Response: Done. We have changed “aboveground litter biomass” to “aboveground litter product”.
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RC1: 'Comment on egusphere-2023-2460', Anonymous Referee #1, 15 Dec 2023
Dear authors,
I carefully read your manuscript titled “Differential vulnerability of mineral associated and particulate organic carbon to nitrogen addition in a subtropical forest” and hereby I provide a review in the hope that it can be informative and useful.
The paper describes the methods employed and presents the outcomes of a nitrogen fertilization experiment in Southern China. The main objective was to test two hypotheses that were based on previous findings. (i) N addition promotes plant biomass and can promote particulate organic carbon accumulation; and (ii) the treatment can result in increased leaching of cations which in its turn can result in lower mineral associated organic carbon. The hypotheses appear to lack objectivity and novelty, based on existing literature. For example, it is a well-known fact that nitrogen is a limiting factor to plant productivity in a considerable fraction of land, and thus, it is expected that nitrogen addition will result in biomass increase. Despite the good presentation of the results and the clear discussion, the experimental design limits the generalization power of the relationships between SOC persistence and vulnerability to N addition. Moreover, the method section is incomplete. Details about the experimental design that are important to the understanding of the research are missing. In the following points the main shortcomings of the manuscript, in my view, will be outlined.
Lack of explanation for central ideas
What is SOC bridging? How do exchangeable cations affect SOC adsorption? These seems to be important mechanisms to understand the proposed trade-off. A more precise explanation is important for the understanding of the manuscript. Also, the authors could improve the discussion by explaining how the findings described in the manuscript can be useful for ecosystem modelling as stated in the abstract.
Study site and methods presentation.
The description of the study area is very simplistic. It would be important for the manuscript robustness if information about the climate (e.g. annual precipitation and temperature) were present. Information about the historical land use of the area is also important.
Regarding the experimental design:
1 – the base rate of N deposition is mentioned but no value is provided.
2 - The randomization procedure of the plots is omitted, leaving the reader with no clue of how pseudo replication was avoided.
3 - The frequency of nitrogen additions and the concentration of the solution used are also omitted, with the authors simply stating the urea solution in deionized was “consistently sprayed”.
4 - The authors mention that they did not observe any marked differences between basic physicochemical properties of the soils of the plots before N additions. If there was a sampling campaign describing the plot conditions before the experiment, these baseline indicators should be presented and explored.
5 - Should we have OC_poc where we have OC_maoc in eq. 2? I have the impression that the utility of the eq. 1 is questionable because its terms are cancelled out in equations 2 and 3. The justification and explanation of the of mass recovery (eq. 1) would improve the robustness of the manuscript.
Methodological issues.
The experimental design resulted in four observations for each treatment. At each sampled plot, five samples were aggregated to form one observation. I wonder if the analysis of variance followed by an LSD test is the optimal approach to deal with the inherent limitations of an experiment of such nature. The authors could explore, for example, the application of linear mixed models using the blocks as a random effects variable, enabling the use of the five samples while considering pseudoreplication effects.
Presentation of the results.
The supplementary material is 12 lines long. I suggest adding it to the main text.
One last detail:
L 38 – 42: The sentence is very long. Also, the sentence starting in line 42 is confusing. I suggest rephrasing it.
Citation: https://doi.org/10.5194/egusphere-2023-2460-RC1 -
AC2: 'Reply on RC1', Qiufang Zhang, 25 Dec 2023
Response to the Comments of Reviewers
I carefully read your manuscript titled “Differential vulnerability of mineral associated and particulate organic carbon to nitrogen addition in a subtropical forest” and hereby I provide a review in the hope that it can be informative and useful. The paper describes the methods employed and presents the outcomes of a nitrogen fertilization experiment in Southern China. The main objective was to test two hypotheses that were based on previous findings. (i) N addition promotes plant biomass and can promote particulate organic carbon accumulation; and (ii) the treatment can result in increased leaching of cations which in its turn can result in lower mineral associated organic carbon. The hypotheses appear to lack objectivity and novelty, based on existing literature. For example, it is a well-known fact that nitrogen is a limiting factor to plant productivity in a considerable fraction of land, and thus, it is expected that nitrogen addition will result in biomass increase. Despite the good presentation of the results and the clear discussion, the experimental design limits the generalization power of the relationships between SOC persistence and vulnerability to N addition. Moreover, the method section is incomplete. Details about the experimental design that are important to the understanding of the research are missing. In the following points the main shortcomings of the manuscript, in my view, will be outlined.
Response: Thank you very much for your kind work and constructive comments and suggestions, which greatly improved the quality of the manuscript. Among major changes in the revised manuscript, (1) we have rewritten the hypothesis. “We hypothesized that 1) N addition promotes POC accumulation, as the N availability induced by N addition increases plant biomass and litter input, and soil acidification leads to a decrease in microbial biomass and enzyme activity; 2) N addition decreases MAOC content, because N addition causes the depletion of exchangeable cations and attenuates mineral protection” (Lines 102–106). (2) Based on the comments of you and Reviewer 1, we have removed the SOC persistence, and changed the title from “Differential vulnerability of mineral-associated and particulate soil organic carbon to nitrogen addition in a subtropical forest” to “Nitrogen addition promotes the accumulation of soil particulate organic carbon in a subtropical forest”. (3) We have added detail description of experimental design in the “Study site” section. Please see Lines 110–135.
Lack of explanation for central ideas
What is SOC bridging? How do exchangeable cations affect SOC adsorption? These seems to be important mechanisms to understand the proposed trade-off. A more precise explanation is important for the understanding of the manuscript. Also, the authors could improve the discussion by explaining how the findings described in the manuscript can be useful for ecosystem modelling as stated in the abstract.
Response: Here, using "SOC bridging" may be a non-standard term, more accurately it should be an “cation bridging”. Cation bridging allows for the interaction of two negatively charged surfaces such as a phyllosilicate and an organic anion. SOC can be stabilized through sorptive interactions. These interactions include sorption to minerals, like phyllosilicate clays, Al-, Fe-, Mn-oxides, poorly crystalline minerals, or polyvalent cations forming bridges to mineral or other organic soil constituents (Rowley et al., 2018). We have provided explanations in the new manuscript. Please see Lines 88–93, 294–298. In addition, “the response of different SOC functional fractions to N addition is inconsistent. However, current ecosystem models mainly consider the bulk soil. In the future, incorporating different SOC functional fractions into ecosystem models will help more accurately predict SOC dynamics under climate change (Abramoff et al., 2022)” (Lines 322–326). We have included this in the discussion to align it with the abstract.
Study site and methods presentation.
The description of the study area is very simplistic. It would be important for the manuscript robustness if information about the climate (e.g. annual precipitation and temperature) were present. Information about the historical land use of the area is also important.
Response: We agree with your opinions. We have added detail description of experimental design in the “Study site” section. A Castanopsis fabri natural forest in Daiyun Mountain National Nature Reserve in southern China's has been selected as an experimental area. The study site has a typical subtropical oceanic monsoon climate. The reserve's average annual temperature and rainfall are roughly 17.6 °C and 1850 mm, respectively, and precipitation mainly occurs during March–September. Since the establishment of the national nature reserve, this forest has not been disturbed by human activities. At the beginning of the experiment, the tree height was 15-20 m, the diameter at breast height was 20-40 cm, and the closure was about 75%. Please see Lines 110–120.
Regarding the experimental design:
1 – the base rate of N deposition is mentioned but no value is provided.
Response: Based on previous reports on atmospheric N deposition rates in the study and nearby areas (Zhou and Yan, 2001; Yuan et al., 2016), we set up the following experimental treatments: control (CT, +0 kg N ha−1 yr−1), low-N addition (LN, +40 kg N ha−1 yr−1), and high-N addition (HN, +80 kg N ha−1 yr−1). Please see Lines 124–128.
2 - The randomization procedure of the plots is omitted, leaving the reader with no clue of how pseudo replication was avoided.
Response: We have added a description of the experimental design to avoid misunderstandings for readers. Please see Lines 123–135. “In December 2019, a total of twelve 10 m × 10 m plots were established. Spacing between plots was > 10 m to avoid N fertilizer transfer. There was no significant difference in soil organic carbon (SOC), total N, and pH among different plots before N addition. The experiment adopted a completely random design, and the three N addition treatments were randomly distributed in twelve plots with four replicates per treatment”.
3 - The frequency of nitrogen additions and the concentration of the solution used are also omitted, with the authors simply stating the urea solution in deionized was “consistently sprayed”.
Response: “Starting in early May 2020, N fertilizer was added once a month during March to September. A specific amount of urea (CO(NH2)2; LN, 24.49 g; HN, 48.98 g) was dissolved in 20 L deionized water and uniformly sprayed over the low-N addition and high-N addition plots using a backpack sprayer. For the control plots, the same volume of deionized water was sprayed”. Please see Lines 131–135.
4 - The authors mention that they did not observe any marked differences between basic physicochemical properties of the soils of the plots before N additions. If there was a sampling campaign describing the plot conditions before the experiment, these baseline indicators should be presented and explored.
Response: There was no significant difference in soil organic carbon (SOC), total N (TN), and pH among different plots before N addition (Table R1)
Table R1 Basic properties of soil at the beginning of experiment
Index
CT
LN
HN
P
SOC (g kg-1)
53.94 ± 5.29
57.29 ± 6.42
53.31 ± 2.76
> 0.05
TN (g kg-1)
3.13 ± 0.11
3.08 ± 0.07
3.07 ± 0.04
> 0.05
pH
5.12 ± 0.11
5.08 ± 0.13
5.10 ± 0.10
> 0.05
CT, control; LN, low-nitrogen addition; HN, high-nitrogen addition; One-way analysis of variance was performed to determine the effects of N addition on basic soil physiochemical properties (α = 0.05). P represents the main effect of N addition.
5 - Should we have OC_poc where we have OC_maoc in eq. 2? I have the impression that the utility of the eq. 1 is questionable because its terms are cancelled out in equations 2 and 3. The justification and explanation of the of mass recovery (eq. 1) would improve the robustness of the manuscript.
Response: We have corrected the eq. 2. Please see Line 170.
Methodological issues.
The experimental design resulted in four observations for each treatment. At each sampled plot, five samples were aggregated to form one observation. I wonder if the analysis of variance followed by an LSD test is the optimal approach to deal with the inherent limitations of an experiment of such nature. The authors could explore, for example, the application of linear mixed models using the blocks as a random effects variable, enabling the use of the five samples while considering pseudoreplication effects.
Response: We have added a description of the experimental design to avoid misunderstandings for readers. Please see Lines 123–135.The experiment adopted a completely random design, and the three N addition treatments were randomly distributed in twelve plots with four replicates per treatment, so LSD test is the optimal approach.
Presentation of the results.
The supplementary material is 12 lines long. I suggest adding it to the main text.
Response: Done. The data is presented in Table 1.
One last detail:
L38-42: The sentence is very long. Also, the sentence starting in line 42 is confusing. I suggest rephrasing it.
Response: Done. Please see Lines 36–41.
References
Abramoff, R. Z., Guenet, B., Zhang, H., Georgiou, K., Xu, X., Viscarra Rossel, R. A., Yuan, W., Ciais, P.: Improved global-scale predictions of soil carbon stocks with Millennial Version 2. Soil Biol. Biochem., 164, 108466, https://doi.org/10.1016/j.soilbio.2021.108466, 2022.
Rowley, M. C., Grand, S., Verrecchia, É. P.: Calcium− mediated stabilisation of soil organic carbon. Biogeochemistry, 137, 27–49. https://doi.org/10.1007/s10533-017-0410-1, 2018.
Yuan, L., Li, W., Chen, W., Zhang, J., Cai, Z.: Characteristics of nitrogen deposition in Daiyun Mountain National Nature Reserve, Environmental Science, 37, 4142–4146, https://doi.org/10.13227/j.hjkx.201605184, 2016.
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AC2: 'Reply on RC1', Qiufang Zhang, 25 Dec 2023
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RC2: 'Comment on egusphere-2023-2460', Anonymous Referee #2, 18 Dec 2023
Chen and the co-authors conducted a N addition experiments in subtropical region to investigate the responses of POC and MAOC to N addition and as well as the underling mechanisms. They found that 1.5-year N addition significantly increased POC content, but had no effect on MAOC. Consequently, the ratio of MAOC to POC was decreased in N addition plots. The increased POC was positively associated with the increasing input of plant litter. Their findings are of course important for the assessment of effect of N deposition on C pools. The manuscript was well prepared and easy to read. However, I have some concerns need to be addressed before this manuscript could be accepted.
First, the novelty of this experiment or the finding of this manuscript needs to be highlighted.
Second, the duration of N addition was two short (only 1.5 years). Of course, several properties that were susceptible to N addition may be changed under N addition, such as the activities of soil enzymes and the contents of exchangeable cations. However, plant litter input change also significantly affected by N addition in such short term. Could you please provide the monthly data of plant litter input for different N addition treatments?
Third, the non-significant response of MAOC to N addition may be mainly due to the duration of N addition was too short.
Forth, the information about the background N deposition rate need to be provided, because this information was tightly linked to the reason for the choosing of 40 and 80 kg N ha-1 yr-1, respectively, as low and high N addition.
Citation: https://doi.org/10.5194/egusphere-2023-2460-RC2 -
AC3: 'Reply on RC2', Qiufang Zhang, 25 Dec 2023
Response to the Comments of Reviewers
Chen and the co-authors conducted a N addition experiments in subtropical region to investigate the responses of POC and MAOC to N addition and as well as the underling mechanisms. They found that 1.5-year N addition significantly increased POC content, but had no effect on MAOC. Consequently, the ratio of MAOC to POC was decreased in N addition plots. The increased POC was positively associated with the increasing input of plant litter. Their findings are of course important for the assessment of effect of N deposition on C pools. The manuscript was well prepared and easy to read. However, I have some concerns need to be addressed before this manuscript could be accepted.
Response: Thank you very much for your kind work and constructive comments and suggestions, which greatly improved the quality of the manuscript.
First, the novelty of this experiment or the finding of this manuscript needs to be highlighted.
Response: In the present study, the main finding is that the response of different SOC functional fractions to N addition is inconsistent. Specifically, short-term N addition has a significant positive effect on POC, which is attributed to the combination of increased plant litter input and decreased microbial decomposition. By contrast, the mineral sorption and desorption reactions jointly led to no significant response of MAOC to N addition. We have highlighted in the new manuscript.
Second, the duration of N addition was two short (only 1.5 years). Of course, several properties that were susceptible to N addition may be changed under N addition, such as the activities of soil enzymes and the contents of exchangeable cations. However, plant litter input change also significantly affected by N addition in such short term. Could you please provide the monthly data of plant litter input for different N addition treatments?
Response: The monthly input data of plant litter under different N application treatments are shown in the Figure R1. Litter from the same plot (with three traps) was collected and mixed in the laboratory. The collected litter was dried and weighed to determine the aboveground litter product.
Third, the non-significant response of MAOC to N addition may be mainly due to the duration of N addition was too short.
Response: We agree with your point of view and have added this reason to the new manuscript. Please see Lines 288–290.
Forth, the information about the background N deposition rate need to be provided, because this information was tightly linked to the reason for the choosing of 40 and 80 kg N ha-1 yr-1, respectively, as low and high N addition.
Response: The low and high here are equivalent in this manuscript. Yuan et al. (2016) investigate the background value of local nitrogen deposition flux in Daiyun Mountain National Nature Reserve of Fujian province using N deposition collector. The results showed that N deposition of Daiyun Mountain National Nature Reserve was 17 kg ha−1 from March to October in 2015. In addition, we also combined the N deposition in other regions, such as Dinghu Mountain in South Asian tropics (38 kg N ha−1 yr−1; Zhou and Yan, 2001). Finally, we set up the following experimental treatments: control (CT, +0 kg N ha−1 yr−1), low-N addition (LN, +40 kg N ha−1 yr−1), and high-N addition (HN, +80 kg N ha−1 yr−1). We have added the information about the background N deposition rate. Please see Lines 123–135.
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AC3: 'Reply on RC2', Qiufang Zhang, 25 Dec 2023
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RC3: 'Comment on egusphere-2023-2460', Anonymous Referee #3, 19 Dec 2023
Overall, I think the manuscript content of this article lacks novelty and innovation, the language logic is confusing, and the article is less readable. The experimental design failed to be clearly and unambiguously expressed; the description of the results and analyses section was too simple.
Unfortunately, I cannot recommend it to publish at current stage.
Hope my comments help authors to improve the manuscript.Citation: https://doi.org/10.5194/egusphere-2023-2460-RC3 -
AC4: 'Reply on RC3', Qiufang Zhang, 25 Dec 2023
Response to the Comments of Reviewers
Overall, I think the manuscript content of this article lacks novelty and innovation, the language logic is confusing, and the article is less readable. The experimental design failed to be clearly and unambiguously expressed; the description of the results and analyses section was too simple.
Unfortunately, I cannot recommend it to publish at current stage.
Hope my comments help authors to improve the manuscript.
Response: Thank you very much for your kind work and constructive comments and suggestions, which greatly improved the quality of the manuscript. In the revised manuscript, (1) the main finding is that the response of different SOC functional fractions to N addition is inconsistent. We have highlighted it. (2) We have invited English professionals to polish this manuscript. (3) We have added detail description of experimental design in the “Study site” section. Please see Lines 110–135. (4) We also tried other analyses (i.e., structural equation model, random forest), but the results were not ideal, so they were not included in this study.
Citation: https://doi.org/10.5194/egusphere-2023-2460-AC4
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AC4: 'Reply on RC3', Qiufang Zhang, 25 Dec 2023
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EC1: 'Comment on egusphere-2023-2460', Sebastian Leuzinger, 14 Jan 2024
I feel that the authors addressed the concerns of the reviewers well. The novelty might be a little marginal, but it is clearly highlighted now, and the methods are explained well. I recommend publication.
Citation: https://doi.org/10.5194/egusphere-2023-2460-EC1 -
EC2: 'Comment on egusphere-2023-2460', Sebastian Leuzinger, 19 Jan 2024
Revising my earlier comment (sorry this is my first time as a guest editor and I am not used to this type of peer review). Based on the one community comment and the three reviews, I recommend rejection of the manuscript. On the one hand, as the reviewers clearly point out, the conceptual idea lacks novelty an is largely confirmatory. On the other hand, the experimental design is too minimal in space and time to yield robust and interpretable results: Only four true replicates exist, as the samples per plot were physically combined. The fertiliser application happened on one single occasion, and the sampling took place more than a year later. It is unclear why this time frame was chosen, and the treatment hardly mimics long term N deposition.
Citation: https://doi.org/10.5194/egusphere-2023-2460-EC2
Status: closed
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CC1: 'Comment on egusphere-2023-2460', Yalin Hu, 20 Nov 2023
The authors in this paper reported the effects of nitrogen addition on soil particulate (POC) and mineral-associated organic carbon (MAOC) content after a short time in one subtropical forest. The results showed that POC content increased by N addition, but there was no change of MAOC content. Furthermore, the authors stated the increases of aboveground litter production and lowered soil enzymatic activities mainly explained the POC content, and the trade-off of exchangeable cations resulted in no changes of MAOC content. This study can provide us some information on the influences of N deposition on SOC. However, there are many previous studies that have been reported the effects of N addition on soil C pools and the chemical exchangeable cations. Thus it seemed that novelty of this study is slightly weak, especially considering that only one Castanopsis fabri forest stand and it is not the most typical vegetation in the studied sub-tropic region. More detail minor comments were as follows
- Line 42-43, this sentence is not clear and not robust conclusion. Based on the data of this study, you can not conclude N addition reduced the persistence of SOC and the less vulnerable MAOC because of the less change of MAOC content. It is also not very clear and difficult to understand “the nutrient density of SOC”
- Study site Section, you need to give more detail description of forest stand, such as natural forest or plantation forest, stand age, tree density, and so on.
- Experimental design, it is not robust experiment considering just one time of N addition after 1.5 year when you collected soil sample in December 2021. Moreover, the author determined BG and CBH enzymatic activities just related to cellulose decomposition.
- Result section, SOC content and soil pH values should be necessary and important data and it is better to give these data in text.
- Line 285-286, it is too speculated that the less root biomass explained the lack of MAOC content.
- Line 318-320, it is not reasonable to conclude N addition reduced the persistence of SOC based on the lower MAOC:POC ratio.
- 4, change “aboveground litter biomass” to “ aboveground litter product”
Citation: https://doi.org/10.5194/egusphere-2023-2460-CC1 -
AC1: 'Reply on CC1', Qiufang Zhang, 07 Dec 2023
The authors in this paper reported the effects of nitrogen addition on soil particulate (POC) and mineral-associated organic carbon (MAOC) content after a short time in one subtropical forest. The results showed that POC content increased by N addition, but there was no change of MAOC content. Furthermore, the authors stated the increases of aboveground litter production and lowered soil enzymatic activities mainly explained the POC content, and the trade-off of exchangeable cations resulted in no changes of MAOC content. This study can provide us some information on the influences of N deposition on SOC. However, there are many previous studies that have been reported the effects of N addition on soil C pools and the chemical exchangeable cations. Thus it seemed that novelty of this study is slightly weak, especially considering that only one Castanopsis fabri forest stand and it is not the most typical vegetation in the studied sub-tropic region. More detail minor comments were as follows.
Response: Thank you very much for your kind work and constructive comments, which greatly improved the quality of the manuscript.
Line 42-43, this sentence is not clear and not robust conclusion. Based on the data of this study, you can not conclude N addition reduced the persistence of SOC and the less vulnerable MAOC because of the less change of MAOC content. It is also not very clear and difficult to understand “the nutrient density of SOC”.
Response: We have revised the sentence as “the accumulation of SOC under N addition is primarily driven by POC, and POC with less protection is more vulnerable to N addition than MAOC with more protection”. Please see Lines 40–42.
Study site Section, you need to give more detail description of forest stand, such as natural forest or plantation forest, stand age, tree density, and so on.
Response: This is a natural forest. Since the establishment of the national nature reserve, this forest has not been disturbed by human activities. At the beginning of the experiment, the tree height was 15-20 meters, the diameter at breast height was 20-40 centimeters, and the closure was about 75%. We have added those detail description of forest in the “Study site” section. Please see Lines 114–122.
Experimental design, it is not robust experiment considering just one time of N addition after 1.5 year when you collected soil sample in December 2021. Moreover, the author determined BG and CBH enzymatic activities just related to cellulose decomposition.
Response: We agree with your point of view. We will also focus on the effects of longer periods of N addition in later experiments. BG and CBH are both cellulase enzymes that act on the ends of reductive and non-reductive cellulose polysaccharide chains to release glucose or cellobiose. BG hydrolyzes cellobiose to produce two molecules of glucose. We originally considered enzymes such as polyphenol oxidase and peroxidase for degrading polyphenolic compounds, but short-term N addition had no significant effect on these two enzymes. This may be because short-term N addition did not significantly promote the accumulation of recalcitrant organic carbon in the soil, so it was not included in this study.
Result section, SOC content and soil pH values should be necessary and important data and it is better to give these data in text.
Response: Done. The data is presented in Table 1. Compared with the control, both low-N addition and high-N addition significantly reduced soil pH but significantly increased SOC content. Please see Lines 216–217.
Line 285-286, it is too speculated that the less root biomass explained the lack of MAOC content.
Response: Thank you for your kind suggestion. We have changed the sentence to “fine root biomass was not influenced by short-term N addition in this study, and there was no significant relationship between fine root biomass and MAOM. Therefore, the regulatory role of fine root biomass on MAOM needs further exploration”. Please see Lines 294–296.
Line 318-320, it is not reasonable to conclude N addition reduced the persistence of SOC based on the lower MAOC:POC ratio.
Response: We agree with your point of view and have deleted it.
Change “aboveground litter biomass” to “aboveground litter product”.
Response: Done. We have changed “aboveground litter biomass” to “aboveground litter product”.
Citation: https://doi.org/10.5194/egusphere-2023-2460-AC1 -
AC5: 'Reply on CC1', Qiufang Zhang, 05 Jan 2024
The authors in this paper reported the effects of nitrogen addition on soil particulate (POC) and mineral-associated organic carbon (MAOC) content after a short time in one subtropical forest. The results showed that POC content increased by N addition, but there was no change of MAOC content. Furthermore, the authors stated the increases of aboveground litter production and lowered soil enzymatic activities mainly explained the POC content, and the trade-off of exchangeable cations resulted in no changes of MAOC content. This study can provide us some information on the influences of N deposition on SOC. However, there are many previous studies that have been reported the effects of N addition on soil C pools and the chemical exchangeable cations. Thus it seemed that novelty of this study is slightly weak, especially considering that only one Castanopsis fabri forest stand and it is not the most typical vegetation in the studied sub-tropic region. More detail minor comments were as follows.
Response: Thank you very much for your kind work and constructive comments, which greatly improved the quality of the manuscript. Based on the comments of other reviewers, we have made revisions to the entire text. In this new response letter, we have revised the line numbers.
Line 42-43, this sentence is not clear and not robust conclusion. Based on the data of this study, you can not conclude N addition reduced the persistence of SOC and the less vulnerable MAOC because of the less change of MAOC content. It is also not very clear and difficult to understand “the nutrient density of SOC”.
Response: We have revised the sentence as “the accumulation of SOC under short-term N addition is primarily driven by POC, and the response of different SOC functional fractions to N addition is inconsistent”. Please see Lines 39–41.
Study site Section, you need to give more detail description of forest stand, such as natural forest or plantation forest, stand age, tree density, and so on.
Response: This is Castanopsis fabri natural forest. Since the establishment of the national nature reserve, this forest has not been disturbed by human activities. At the beginning of the experiment, the tree height was 15-20 meters, the diameter at breast height was 20-40 centimeters, and the closure was about 75%. We have added those detail description of forest in the “Study site” section. Please see Lines 110–118.
Experimental design, it is not robust experiment considering just one time of N addition after 1.5 year when you collected soil sample in December 2021. Moreover, the author determined BG and CBH enzymatic activities just related to cellulose decomposition.
Response: We agree with your opinion. We will focus on the effects of longer periods of N addition in later experiments. BG and CBH are both cellulase enzymes that act on the ends of reductive and non-reductive cellulose polysaccharide chains to release glucose or cellobiose. BG hydrolyzes cellobiose to produce two molecules of glucose. We originally considered enzymes such as polyphenol oxidase and peroxidase for degrading polyphenolic compounds, but short-term N addition had no significant effect on these two enzymes. This may be because short-term N addition did not significantly promote the accumulation of recalcitrant organic carbon in the soil, so it was not included in this study.
Result section, SOC content and soil pH values should be necessary and important data and it is better to give these data in text.
Response: Done. The data is presented in Table 1. Compared with the control, both low-N addition and high-N addition significantly reduced soil pH and significantly increased SOC content. Please see Lines 215–216.
Line 285-286, it is too speculated that the less root biomass explained the lack of MAOC content.
Response: Thank you for your kind suggestion. We have changed the sentence to “fine root biomass was not influenced by short-term N addition in this study, and there was no significant relationship between fine root biomass and MAOC. The regulatory role of fine root biomass on MAOC needs further exploration”. Please see Lines 316–319.
Line 318-320, it is not reasonable to conclude N addition reduced the persistence of SOC based on the lower MAOC:POC ratio.
Response: We agree with your opinion and have deleted it.
Change “aboveground litter biomass” to “aboveground litter product”.
Response: Done. We have changed “aboveground litter biomass” to “aboveground litter product”.
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RC1: 'Comment on egusphere-2023-2460', Anonymous Referee #1, 15 Dec 2023
Dear authors,
I carefully read your manuscript titled “Differential vulnerability of mineral associated and particulate organic carbon to nitrogen addition in a subtropical forest” and hereby I provide a review in the hope that it can be informative and useful.
The paper describes the methods employed and presents the outcomes of a nitrogen fertilization experiment in Southern China. The main objective was to test two hypotheses that were based on previous findings. (i) N addition promotes plant biomass and can promote particulate organic carbon accumulation; and (ii) the treatment can result in increased leaching of cations which in its turn can result in lower mineral associated organic carbon. The hypotheses appear to lack objectivity and novelty, based on existing literature. For example, it is a well-known fact that nitrogen is a limiting factor to plant productivity in a considerable fraction of land, and thus, it is expected that nitrogen addition will result in biomass increase. Despite the good presentation of the results and the clear discussion, the experimental design limits the generalization power of the relationships between SOC persistence and vulnerability to N addition. Moreover, the method section is incomplete. Details about the experimental design that are important to the understanding of the research are missing. In the following points the main shortcomings of the manuscript, in my view, will be outlined.
Lack of explanation for central ideas
What is SOC bridging? How do exchangeable cations affect SOC adsorption? These seems to be important mechanisms to understand the proposed trade-off. A more precise explanation is important for the understanding of the manuscript. Also, the authors could improve the discussion by explaining how the findings described in the manuscript can be useful for ecosystem modelling as stated in the abstract.
Study site and methods presentation.
The description of the study area is very simplistic. It would be important for the manuscript robustness if information about the climate (e.g. annual precipitation and temperature) were present. Information about the historical land use of the area is also important.
Regarding the experimental design:
1 – the base rate of N deposition is mentioned but no value is provided.
2 - The randomization procedure of the plots is omitted, leaving the reader with no clue of how pseudo replication was avoided.
3 - The frequency of nitrogen additions and the concentration of the solution used are also omitted, with the authors simply stating the urea solution in deionized was “consistently sprayed”.
4 - The authors mention that they did not observe any marked differences between basic physicochemical properties of the soils of the plots before N additions. If there was a sampling campaign describing the plot conditions before the experiment, these baseline indicators should be presented and explored.
5 - Should we have OC_poc where we have OC_maoc in eq. 2? I have the impression that the utility of the eq. 1 is questionable because its terms are cancelled out in equations 2 and 3. The justification and explanation of the of mass recovery (eq. 1) would improve the robustness of the manuscript.
Methodological issues.
The experimental design resulted in four observations for each treatment. At each sampled plot, five samples were aggregated to form one observation. I wonder if the analysis of variance followed by an LSD test is the optimal approach to deal with the inherent limitations of an experiment of such nature. The authors could explore, for example, the application of linear mixed models using the blocks as a random effects variable, enabling the use of the five samples while considering pseudoreplication effects.
Presentation of the results.
The supplementary material is 12 lines long. I suggest adding it to the main text.
One last detail:
L 38 – 42: The sentence is very long. Also, the sentence starting in line 42 is confusing. I suggest rephrasing it.
Citation: https://doi.org/10.5194/egusphere-2023-2460-RC1 -
AC2: 'Reply on RC1', Qiufang Zhang, 25 Dec 2023
Response to the Comments of Reviewers
I carefully read your manuscript titled “Differential vulnerability of mineral associated and particulate organic carbon to nitrogen addition in a subtropical forest” and hereby I provide a review in the hope that it can be informative and useful. The paper describes the methods employed and presents the outcomes of a nitrogen fertilization experiment in Southern China. The main objective was to test two hypotheses that were based on previous findings. (i) N addition promotes plant biomass and can promote particulate organic carbon accumulation; and (ii) the treatment can result in increased leaching of cations which in its turn can result in lower mineral associated organic carbon. The hypotheses appear to lack objectivity and novelty, based on existing literature. For example, it is a well-known fact that nitrogen is a limiting factor to plant productivity in a considerable fraction of land, and thus, it is expected that nitrogen addition will result in biomass increase. Despite the good presentation of the results and the clear discussion, the experimental design limits the generalization power of the relationships between SOC persistence and vulnerability to N addition. Moreover, the method section is incomplete. Details about the experimental design that are important to the understanding of the research are missing. In the following points the main shortcomings of the manuscript, in my view, will be outlined.
Response: Thank you very much for your kind work and constructive comments and suggestions, which greatly improved the quality of the manuscript. Among major changes in the revised manuscript, (1) we have rewritten the hypothesis. “We hypothesized that 1) N addition promotes POC accumulation, as the N availability induced by N addition increases plant biomass and litter input, and soil acidification leads to a decrease in microbial biomass and enzyme activity; 2) N addition decreases MAOC content, because N addition causes the depletion of exchangeable cations and attenuates mineral protection” (Lines 102–106). (2) Based on the comments of you and Reviewer 1, we have removed the SOC persistence, and changed the title from “Differential vulnerability of mineral-associated and particulate soil organic carbon to nitrogen addition in a subtropical forest” to “Nitrogen addition promotes the accumulation of soil particulate organic carbon in a subtropical forest”. (3) We have added detail description of experimental design in the “Study site” section. Please see Lines 110–135.
Lack of explanation for central ideas
What is SOC bridging? How do exchangeable cations affect SOC adsorption? These seems to be important mechanisms to understand the proposed trade-off. A more precise explanation is important for the understanding of the manuscript. Also, the authors could improve the discussion by explaining how the findings described in the manuscript can be useful for ecosystem modelling as stated in the abstract.
Response: Here, using "SOC bridging" may be a non-standard term, more accurately it should be an “cation bridging”. Cation bridging allows for the interaction of two negatively charged surfaces such as a phyllosilicate and an organic anion. SOC can be stabilized through sorptive interactions. These interactions include sorption to minerals, like phyllosilicate clays, Al-, Fe-, Mn-oxides, poorly crystalline minerals, or polyvalent cations forming bridges to mineral or other organic soil constituents (Rowley et al., 2018). We have provided explanations in the new manuscript. Please see Lines 88–93, 294–298. In addition, “the response of different SOC functional fractions to N addition is inconsistent. However, current ecosystem models mainly consider the bulk soil. In the future, incorporating different SOC functional fractions into ecosystem models will help more accurately predict SOC dynamics under climate change (Abramoff et al., 2022)” (Lines 322–326). We have included this in the discussion to align it with the abstract.
Study site and methods presentation.
The description of the study area is very simplistic. It would be important for the manuscript robustness if information about the climate (e.g. annual precipitation and temperature) were present. Information about the historical land use of the area is also important.
Response: We agree with your opinions. We have added detail description of experimental design in the “Study site” section. A Castanopsis fabri natural forest in Daiyun Mountain National Nature Reserve in southern China's has been selected as an experimental area. The study site has a typical subtropical oceanic monsoon climate. The reserve's average annual temperature and rainfall are roughly 17.6 °C and 1850 mm, respectively, and precipitation mainly occurs during March–September. Since the establishment of the national nature reserve, this forest has not been disturbed by human activities. At the beginning of the experiment, the tree height was 15-20 m, the diameter at breast height was 20-40 cm, and the closure was about 75%. Please see Lines 110–120.
Regarding the experimental design:
1 – the base rate of N deposition is mentioned but no value is provided.
Response: Based on previous reports on atmospheric N deposition rates in the study and nearby areas (Zhou and Yan, 2001; Yuan et al., 2016), we set up the following experimental treatments: control (CT, +0 kg N ha−1 yr−1), low-N addition (LN, +40 kg N ha−1 yr−1), and high-N addition (HN, +80 kg N ha−1 yr−1). Please see Lines 124–128.
2 - The randomization procedure of the plots is omitted, leaving the reader with no clue of how pseudo replication was avoided.
Response: We have added a description of the experimental design to avoid misunderstandings for readers. Please see Lines 123–135. “In December 2019, a total of twelve 10 m × 10 m plots were established. Spacing between plots was > 10 m to avoid N fertilizer transfer. There was no significant difference in soil organic carbon (SOC), total N, and pH among different plots before N addition. The experiment adopted a completely random design, and the three N addition treatments were randomly distributed in twelve plots with four replicates per treatment”.
3 - The frequency of nitrogen additions and the concentration of the solution used are also omitted, with the authors simply stating the urea solution in deionized was “consistently sprayed”.
Response: “Starting in early May 2020, N fertilizer was added once a month during March to September. A specific amount of urea (CO(NH2)2; LN, 24.49 g; HN, 48.98 g) was dissolved in 20 L deionized water and uniformly sprayed over the low-N addition and high-N addition plots using a backpack sprayer. For the control plots, the same volume of deionized water was sprayed”. Please see Lines 131–135.
4 - The authors mention that they did not observe any marked differences between basic physicochemical properties of the soils of the plots before N additions. If there was a sampling campaign describing the plot conditions before the experiment, these baseline indicators should be presented and explored.
Response: There was no significant difference in soil organic carbon (SOC), total N (TN), and pH among different plots before N addition (Table R1)
Table R1 Basic properties of soil at the beginning of experiment
Index
CT
LN
HN
P
SOC (g kg-1)
53.94 ± 5.29
57.29 ± 6.42
53.31 ± 2.76
> 0.05
TN (g kg-1)
3.13 ± 0.11
3.08 ± 0.07
3.07 ± 0.04
> 0.05
pH
5.12 ± 0.11
5.08 ± 0.13
5.10 ± 0.10
> 0.05
CT, control; LN, low-nitrogen addition; HN, high-nitrogen addition; One-way analysis of variance was performed to determine the effects of N addition on basic soil physiochemical properties (α = 0.05). P represents the main effect of N addition.
5 - Should we have OC_poc where we have OC_maoc in eq. 2? I have the impression that the utility of the eq. 1 is questionable because its terms are cancelled out in equations 2 and 3. The justification and explanation of the of mass recovery (eq. 1) would improve the robustness of the manuscript.
Response: We have corrected the eq. 2. Please see Line 170.
Methodological issues.
The experimental design resulted in four observations for each treatment. At each sampled plot, five samples were aggregated to form one observation. I wonder if the analysis of variance followed by an LSD test is the optimal approach to deal with the inherent limitations of an experiment of such nature. The authors could explore, for example, the application of linear mixed models using the blocks as a random effects variable, enabling the use of the five samples while considering pseudoreplication effects.
Response: We have added a description of the experimental design to avoid misunderstandings for readers. Please see Lines 123–135.The experiment adopted a completely random design, and the three N addition treatments were randomly distributed in twelve plots with four replicates per treatment, so LSD test is the optimal approach.
Presentation of the results.
The supplementary material is 12 lines long. I suggest adding it to the main text.
Response: Done. The data is presented in Table 1.
One last detail:
L38-42: The sentence is very long. Also, the sentence starting in line 42 is confusing. I suggest rephrasing it.
Response: Done. Please see Lines 36–41.
References
Abramoff, R. Z., Guenet, B., Zhang, H., Georgiou, K., Xu, X., Viscarra Rossel, R. A., Yuan, W., Ciais, P.: Improved global-scale predictions of soil carbon stocks with Millennial Version 2. Soil Biol. Biochem., 164, 108466, https://doi.org/10.1016/j.soilbio.2021.108466, 2022.
Rowley, M. C., Grand, S., Verrecchia, É. P.: Calcium− mediated stabilisation of soil organic carbon. Biogeochemistry, 137, 27–49. https://doi.org/10.1007/s10533-017-0410-1, 2018.
Yuan, L., Li, W., Chen, W., Zhang, J., Cai, Z.: Characteristics of nitrogen deposition in Daiyun Mountain National Nature Reserve, Environmental Science, 37, 4142–4146, https://doi.org/10.13227/j.hjkx.201605184, 2016.
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AC2: 'Reply on RC1', Qiufang Zhang, 25 Dec 2023
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RC2: 'Comment on egusphere-2023-2460', Anonymous Referee #2, 18 Dec 2023
Chen and the co-authors conducted a N addition experiments in subtropical region to investigate the responses of POC and MAOC to N addition and as well as the underling mechanisms. They found that 1.5-year N addition significantly increased POC content, but had no effect on MAOC. Consequently, the ratio of MAOC to POC was decreased in N addition plots. The increased POC was positively associated with the increasing input of plant litter. Their findings are of course important for the assessment of effect of N deposition on C pools. The manuscript was well prepared and easy to read. However, I have some concerns need to be addressed before this manuscript could be accepted.
First, the novelty of this experiment or the finding of this manuscript needs to be highlighted.
Second, the duration of N addition was two short (only 1.5 years). Of course, several properties that were susceptible to N addition may be changed under N addition, such as the activities of soil enzymes and the contents of exchangeable cations. However, plant litter input change also significantly affected by N addition in such short term. Could you please provide the monthly data of plant litter input for different N addition treatments?
Third, the non-significant response of MAOC to N addition may be mainly due to the duration of N addition was too short.
Forth, the information about the background N deposition rate need to be provided, because this information was tightly linked to the reason for the choosing of 40 and 80 kg N ha-1 yr-1, respectively, as low and high N addition.
Citation: https://doi.org/10.5194/egusphere-2023-2460-RC2 -
AC3: 'Reply on RC2', Qiufang Zhang, 25 Dec 2023
Response to the Comments of Reviewers
Chen and the co-authors conducted a N addition experiments in subtropical region to investigate the responses of POC and MAOC to N addition and as well as the underling mechanisms. They found that 1.5-year N addition significantly increased POC content, but had no effect on MAOC. Consequently, the ratio of MAOC to POC was decreased in N addition plots. The increased POC was positively associated with the increasing input of plant litter. Their findings are of course important for the assessment of effect of N deposition on C pools. The manuscript was well prepared and easy to read. However, I have some concerns need to be addressed before this manuscript could be accepted.
Response: Thank you very much for your kind work and constructive comments and suggestions, which greatly improved the quality of the manuscript.
First, the novelty of this experiment or the finding of this manuscript needs to be highlighted.
Response: In the present study, the main finding is that the response of different SOC functional fractions to N addition is inconsistent. Specifically, short-term N addition has a significant positive effect on POC, which is attributed to the combination of increased plant litter input and decreased microbial decomposition. By contrast, the mineral sorption and desorption reactions jointly led to no significant response of MAOC to N addition. We have highlighted in the new manuscript.
Second, the duration of N addition was two short (only 1.5 years). Of course, several properties that were susceptible to N addition may be changed under N addition, such as the activities of soil enzymes and the contents of exchangeable cations. However, plant litter input change also significantly affected by N addition in such short term. Could you please provide the monthly data of plant litter input for different N addition treatments?
Response: The monthly input data of plant litter under different N application treatments are shown in the Figure R1. Litter from the same plot (with three traps) was collected and mixed in the laboratory. The collected litter was dried and weighed to determine the aboveground litter product.
Third, the non-significant response of MAOC to N addition may be mainly due to the duration of N addition was too short.
Response: We agree with your point of view and have added this reason to the new manuscript. Please see Lines 288–290.
Forth, the information about the background N deposition rate need to be provided, because this information was tightly linked to the reason for the choosing of 40 and 80 kg N ha-1 yr-1, respectively, as low and high N addition.
Response: The low and high here are equivalent in this manuscript. Yuan et al. (2016) investigate the background value of local nitrogen deposition flux in Daiyun Mountain National Nature Reserve of Fujian province using N deposition collector. The results showed that N deposition of Daiyun Mountain National Nature Reserve was 17 kg ha−1 from March to October in 2015. In addition, we also combined the N deposition in other regions, such as Dinghu Mountain in South Asian tropics (38 kg N ha−1 yr−1; Zhou and Yan, 2001). Finally, we set up the following experimental treatments: control (CT, +0 kg N ha−1 yr−1), low-N addition (LN, +40 kg N ha−1 yr−1), and high-N addition (HN, +80 kg N ha−1 yr−1). We have added the information about the background N deposition rate. Please see Lines 123–135.
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AC3: 'Reply on RC2', Qiufang Zhang, 25 Dec 2023
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RC3: 'Comment on egusphere-2023-2460', Anonymous Referee #3, 19 Dec 2023
Overall, I think the manuscript content of this article lacks novelty and innovation, the language logic is confusing, and the article is less readable. The experimental design failed to be clearly and unambiguously expressed; the description of the results and analyses section was too simple.
Unfortunately, I cannot recommend it to publish at current stage.
Hope my comments help authors to improve the manuscript.Citation: https://doi.org/10.5194/egusphere-2023-2460-RC3 -
AC4: 'Reply on RC3', Qiufang Zhang, 25 Dec 2023
Response to the Comments of Reviewers
Overall, I think the manuscript content of this article lacks novelty and innovation, the language logic is confusing, and the article is less readable. The experimental design failed to be clearly and unambiguously expressed; the description of the results and analyses section was too simple.
Unfortunately, I cannot recommend it to publish at current stage.
Hope my comments help authors to improve the manuscript.
Response: Thank you very much for your kind work and constructive comments and suggestions, which greatly improved the quality of the manuscript. In the revised manuscript, (1) the main finding is that the response of different SOC functional fractions to N addition is inconsistent. We have highlighted it. (2) We have invited English professionals to polish this manuscript. (3) We have added detail description of experimental design in the “Study site” section. Please see Lines 110–135. (4) We also tried other analyses (i.e., structural equation model, random forest), but the results were not ideal, so they were not included in this study.
Citation: https://doi.org/10.5194/egusphere-2023-2460-AC4
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AC4: 'Reply on RC3', Qiufang Zhang, 25 Dec 2023
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EC1: 'Comment on egusphere-2023-2460', Sebastian Leuzinger, 14 Jan 2024
I feel that the authors addressed the concerns of the reviewers well. The novelty might be a little marginal, but it is clearly highlighted now, and the methods are explained well. I recommend publication.
Citation: https://doi.org/10.5194/egusphere-2023-2460-EC1 -
EC2: 'Comment on egusphere-2023-2460', Sebastian Leuzinger, 19 Jan 2024
Revising my earlier comment (sorry this is my first time as a guest editor and I am not used to this type of peer review). Based on the one community comment and the three reviews, I recommend rejection of the manuscript. On the one hand, as the reviewers clearly point out, the conceptual idea lacks novelty an is largely confirmatory. On the other hand, the experimental design is too minimal in space and time to yield robust and interpretable results: Only four true replicates exist, as the samples per plot were physically combined. The fertiliser application happened on one single occasion, and the sampling took place more than a year later. It is unclear why this time frame was chosen, and the treatment hardly mimics long term N deposition.
Citation: https://doi.org/10.5194/egusphere-2023-2460-EC2
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