Net Carbon Balance between Priming and Replenishment of Soil Organic Carbon with Biochar Addition Regulated by N Addition Differing in Contrasting Forest Ecosystems
Ling Zhao
Round 1
Reviewer 1 Report
This study investigated the net carbon balance between priming and replenishment of soil organic carbon with biochar addition in the context of atmospheric N deposition. They conducted a 365-day incubation experiment with adding 13C-labelled biochar plus five rates of inorganic N (0 to 15% N of soil total N) in in three different forest ecosystems. They concluded that biochar input can contribute to soil C sequestration and that N addition can enhance the C sequestration potential of biochar. This is an interesting and attractive topic which falls in the field of this Journal. The experimental methods were reasonable and the discussions were insightful. The manuscript was well written with a clear structure and very good language. I recommend to accept it with a minor revision.
1) Section 2.1, biochar production: Do you have any considerations for selecting this type of biomass and the pyrolysis temperature of 380oC? Please give a brief explanation in the text.
2) Section 2.2. “The biochar was added as 5% of the SOC concentration”. I am still not very clear about the biochar addition rate. Please give a more clarified statement.
3) “200 ml” should be “200 mL”.
4) Section 4.1 discussion: Authors only mentioned the sorption of soil-derived DOC by biochar leading to a stronger negative PE. I think the mechanisms might be more complex. The interaction of biochar with the compositions of different forest soils may be influence the soil microbes. Perhaps authors should consider these effects.
5) I am still struggling to understand these statements “Therefore, In DBF, N addition would relieve the microorganisms from N limitation and reduce the mining of N from native SOC, thereby enhancing the negative PE. On the contrary, in EBF and ECF, N addition may ulteriorly aggravate microbial C limitation, which may compel microorganisms to decompose more SOC to obtain energy, and consequently an increased positive PE in the early period”. Why N addition did not induce more decomposition of carbon both derived from the biochar and soil?
6) It is suggested that the microbiological community analysis could be performed in the future studies.
Author Response
This study investigated the net carbon balance between priming and replenishment of soil organic carbon with biochar addition in the context of atmospheric N deposition. They conducted a 365-day incubation experiment with adding 13C-labelled biochar plus five rates of inorganic N (0 to 15% N of soil total N) in in three different forest ecosystems. They concluded that biochar input can contribute to soil C sequestration and that N addition can enhance the C sequestration potential of biochar. This is an interesting and attractive topic which falls in the field of this Journal. The experimental methods were reasonable and the discussions were insightful. The manuscript was well written with a clear structure and very good language. I recommend to accept it with a minor revision.
Response: Thank you very much for your recognition of our work.
（1）Section 2.1, biochar production: Do you have any considerations for selecting this type of biomass and the pyrolysis temperature of 380oC? Please give a brief explanation in the text.
Response: Chinese fir is a major timber species that has been extensively planted in provinces of southern China，including our experiment area. In plantation management, managers usually use slash burning to deal with the cutting residues, thus producing a large amount of biochar in soils, which may have significant effects on soil C and nutrient cycling. Therefore, in our experiment, we used 13C-labeled seedling stems of Chinese fir as the parent material of biochar. It has been reported that approximately 94% of forest fires are surface fires and the temperature of forest surface fires is about 350-400 °C. Therefore, to get closer to the biochar under field conditions, we used 380 °C to produce the biochar. As suggested, we give a brief explanation in the revised manuscript (Please see line 103-106 and 112-113).
（2）Section 2.2. “The biochar was added as 5% of the SOC concentration”. I am still not very clear about the biochar addition rate. Please give a more clarified statement.
Response: This sentence is not clear and we rephrased these sentences for improved clarity in the revised manuscript (Please see line 123).
（3）“200 ml” should be “200 mL”.
Response: Thanks, change made.
（4）Section 4.1 discussion: Authors only mentioned the sorption of soil-derived DOC by biochar leading to a stronger negative PE. I think the mechanisms might be more complex. The interaction of biochar with the compositions of different forest soils may be influence the soil microbes. Perhaps authors should consider these effects.
Response: We quite agree with the reviewer’ comments and suggestions. In the revised manuscript, we add the description of these effects.
Additionally, some studies have suggested that the divergent interactions between biochar particles and the compositions (e.g., soil organic matter and clay minerals) of different soils may also result in different PE responses [44,45]. In the present study, given the substantial differences in soil properties (e.g., clay and nutrients contents) and soil microbes (e.g., microbial biomass N) between the three studied soils, it is reasonable to speculate that this mechanism may also act on our results. However, in this study, we did not measure the relevant indicators, and thus this speculation needs further verification.
（5）I am still struggling to understand these statements “Therefore, In DBF, N addition would relieve the microorganisms from N limitation and reduce the mining of N from native SOC, thereby enhancing the negative PE. On the contrary, in EBF and ECF, N addition may ulteriorly aggravate microbial C limitation, which may compel microorganisms to decompose more SOC to obtain energy, and consequently an increased positive PE in the early period”. Why N addition did not induce more decomposition of carbon both derived from the biochar and soil?
Response: In our study, we first calculate the ratio of resource C:N (DOC to mineral N) to microbial biomass C:N to reflect the C:N imbalance between resources and microorganism for each forest soil. In general, a lower C:N imbalance implies higher N availability relative to C availability and could thereafter be used as an agent of microbial N limitation; that is, the higher the C:N imbalance, the more severe the microbial N limitation. In terms of the three forest soils we studied, the microbial C:N imbalance in DBF was about 2-4 times higher than in EBF and ECF (Table 1), therefore , we can think that microbial growth in DBF was more limited by N availability; in turn, microbial growth in EBF and ECF was more limited by C availability. Therefore, in DBF, relative to the sole biochar addition treatment, according to the N mining theory, N addition may relieve the microorganisms from N limitation and thus reduce the mining of N from native SOC (i.e., decrease the mineralization of native SOC), thereby enhancing the negative PE. However, in EBF and ECF where microbial growth was limited by C availability, N addition may ulteriorly aggravate microbial C limitation, compelling microorganisms to decompose more native SOC to obtain energy, and consequently increase the mineralization of native SOC (i.e., positive PE). With regard to why N addition did not induce more decomposition of carbon both derived from the biochar and soil, we think that in DBF as we discussed, biochar may absorb a large amount of soil-derived DOC into biochar pores, leaving the recalcitrant SOC for soil microbes to use. Therefore, in the early stage, N addition may enhance the decomposition of the labile C in biochar but not in native SOC.
（6）It is suggested that the microbiological community analysis could be performed in the future studies.
Response: We thank the reviewer’s suggestions and expound the limitation of not performing the microbiological community analysis in the revised manuscript.
Nevertheless, the above discussion was mainly focused on the impacts of soil properties, while the microbial mechanisms of the PE response to biochar and N additions have not yet been addressed. Zheng et al.[16] recently showed that the adding biochar could modulate the soil bacterial community towards low C turnover bacteria taxa, thereby suppressing native SOC decomposition (i.e., negative PE). Therefore, to advance our understanding of the priming processes, the responses of the soil microbial community to biochar and N additions should be the focus of future studies.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The study presented new results on primary and replenishment effect ob C balance of the applied biochar with different rates of mineral nitrogen. The data were obtained via conducting a 365-day laboratory incubation experiment. The methods of the investigation are contemporary and the data were statistcally analyzed.
The recommendation for revisions are:
1. It is necessary to point out the name of soils according to World reference base for soil resources 2014. You can not: comment that the soils are contrasting assuming only the vegetation cover (line 70 and 71). The texture class has to be included in Table 1. More discussions about the soil properties are needed.
2. The English language can be improved.
Author Response
The study presented new results on primary and replenishment effect ob C balance of the applied biochar with different rates of mineral nitrogen. The data were obtained via conducting a 365-day laboratory incubation experiment. The methods of the investigation are contemporary and the data were statistcally analyzed.
Response: We give thanks to reviewer 2 for a positive evaluation on our job. We also thank his/her suggestions which are very valuable for us to improve this manuscript.
1. It is necessary to point out the name of soils according to World reference base for soil resources 2014
Response: Thanks for this nice suggestion, we have now added the “soil type” in Table 1.
2. You can not: comment that the soils are contrasting assuming only the vegetation cover (line 70 and 71).
Response: We fully accepted the reviewer’ comment. In the revised manuscript, we used soil properties to indicate that the three forest soils are contrasting (Please see lines 72-73).
3. The texture class has to be included in Table 1.
Response: Thanks for this suggestion, we have now added the “soil texture” in Table 1.
4. More discussions about the soil properties are needed.
Response: We thank the reviewer’s suggestions, and we discussed another potential mechanism, that is, the interactions between biochar particles and the compositions of soil, that may result in different PE responses in different forest soils in the revised manuscript.
5. The English language can be improved.
Response: we thank reviewer’s suggestions and employ a professional English-language editing service to check and polish the language, by correcting spelling and grammatical errors, and thus we believe, the revised version is greatly improved.
Author Response File: Author Response.docx
