Exogenous Carbon Addition Reduces Soil Organic Carbon: The Effects of Fungi on Soil Carbon Priming Exceed Those of Bacteria on Soil Carbon Sequestration

Round 1

Reviewer 1 Report

The topic of the manuscript aligns well with the scope of Forests. However, the manuscript requires revisions to ensure it is ready for publication. Please find my detailed comments below:

L17: Is it stable carbon or natural abundance carbon? Please clarify this point.

The experiment design needs to be reorganized and clarified. It would be helpful to provide a clear description of the experimental setup, including the type of soil used and whether the topsoil was used for incubation.

L29-30: The role of fungi in breaking down complex substrates and litter is generally well known, and fungal mycelium is essential for aggregate formation. It would be interesting to discuss the relationship between fungal diversity and microbial activities. Additionally, it would be helpful to know if microbial abundance was measured in this study.

L37: Please specify the type of soil as "karst lime soils" to provide more clarity.

L42, 44: When using abbreviations, please spell out the full name when it is first introduced and check for consistency throughout the manuscript.

L51-52: It would be beneficial to define the term "priming effect" at this point to ensure clarity for readers.

L98-99: Please clarify if this statement is a hypothesis and consider rephrasing it for better clarity.

L165: Please provide clarification on the bulk density of the soil core, soil water content, and temperature during the incubation period.

The Results section should be simplified, and it is not necessary to include all values and p-values. Focus on presenting the key findings in a clear and concise manner.

Figures 5-6: It would be helpful to incorporate the regression equations into the figures for better visual representation.

L554: Please avoid using phrases such as "In Fig. 7" and instead directly present the main findings of the research. For example: "The effects of soil fungi on SOC priming significantly exceeded the effects of bacteria on SOC sequestration (R = 0.40 vs. 0.27, P = 0.003 vs. 0.02), leading to an increase in SOC mineralization and decomposition after the addition of exogenous C (Fig. 7)."

Please consider citing the following paper: [https://doi.org/10.1016/j.scitotenv.2022.158274] as it is relevant to the research topic.

Moderate editing of English language required

Author Response

Respected editorial department and chief reviewers of Forests,

Thank you very much for your valuable comments on this manuscript. We have carefully revised the manuscript according to the review opinions, including:(1) according to the requirements of the journal, we unified the standard format and text of the paper;(2) The paper was carefully modified and improved according to the suggestions put forward by the chief reviewers. For details, please refer to the revised document. The amendments to the relevant issues are further explained as follows,

1. L17: Is it stable carbon or natural abundance carbon? Please clarify this point. The experiment design needs to be reorganized and clarified. It would be helpful to provide a clear description of the experimental setup, including the type of soil used and whether the topsoil was used for incubation.

Answer: Thank you very much for your valuable comments on this manuscript. We have changed the“Few studies have examined the changes in SOC due to microbial activity after exogenous C inputs in karst lime soils in China. In this research, the 13C isotope tracer technique was employed to investigate the priming effect on typical lime soil of ¹³C-litter and ¹³C-CaCO₃ through a mineralization-incubation experiment. Samples were collected at 5, 10, 20, 40, 60, and 80 days of incubation and analyzed for SOC mineralization, SOC distribution across fractions (>250 μm, 53～250 μm, and <53 μm), and soil microbial diversity. A control consisting of no exogenous C addition was included.” to “Few studies have examined the changes in natural abundance C due to microbial activity after exogenous C inputs in karst lime soils in China. In this research, the 13C isotope tracer technique was employed to investigate the priming effect of SOC on typical lime soil (0～20 cm) of ¹³C-litter and ¹³C-CaCO₃ through a mineralization-incubation experiment. Samples were collected at 5, 10, 20, 40, 60, and 80 days of incubation and analyzed for SOC mineralization, SOC distribution across fractions (>250 μm, 53～250 μm, and <53 μm), and soil microbial diversity. A control consisting of no exogenous C addition was included.”

2. L29-30: The role of fungi in breaking down complex substrates and litter is generally well known, and fungal mycelium is essential for aggregate formation. It would be interesting to discuss the relationship between fungal diversity and microbial activities. Additionally, it would be helpful to know if microbial abundance was measured in this study.

Answer: Thank you very much for your valuable comments on this manuscript. We are very sorry that we did not measure microbial abundance during the experiment.

3. L37: Please specify the type of soil as "karst lime soils" to provide more clarity.

Answer: Thank you very much for your valuable comments on this manuscript. We have revised and refined the manuscript.

4. L42, 44: When using abbreviations, please spell out the full name when it is first introduced and check for consistency throughout the manuscript.

    Answer: Thank you very much for your valuable comments on this manuscript. We have changed the “CO₂” to “carbon dioxide (CO₂)”, and the full name of the abbreviation on L44 has been written on L14. Meanwhile, we have checked the abbreviation problem of the whole manuscript.

5. L51-52: It would be beneficial to define the term "priming effect" at this point to ensure clarity for readers.

    Answer: Thank you very much for your valuable comments on this manuscript. We added “The addition of C substrate often modifies the rate of soil organic matter (SOM) decomposition. This is known as the priming effect (Li et al., 2022).”

6. L98-99: Please clarify if this statement is a hypothesis and consider rephrasing it for better clarity.

   Answer: Thank you very much for your valuable comments on this manuscript. We have changed the “It is generally believed that exogenous C addition affects the composition of bacterial and fungal communities, and thus soil C mineralization” to “As is known to all, exogenous C addition affects the composition of bacterial and fungal communities, and thus affects the mineralization decomposition of soil C”.

7. L165: Please provide clarification on the bulk density of the soil core, soil water content, and temperature during the incubation period.

Answer: Thank you very much for your valuable comments on this manuscript. We have recorded soil bulk density in Table 2, and soil water content and incubation temperature in L175-177.

8. The Results section should be simplified, and it is not necessary to include all values and p-values. Focus on presenting the key findings in a clear and concise manner.

    Answer: Thank you very much for your valuable comments on this manuscript. We have removed “(LL and CCL, R² = 0.62, P = 0.000 and R² = 0.48, P = 0.001)”, ” (LL and CCL, R² = 0.57, P = 0.000 418 and R² = 0.39, P = 0.005)”, “(LL and CCL, R² = 0.54, P = 0.001 and R² = 0.51, P = 0.001)” and “(LL and CCL, R² = 0.63, P = 0.000 and R² = 0.52, P = 0.001)” from the        Result section.

9. Figures 5-6: It would be helpful to incorporate the regression equations into the figures for better visual representation.

    Answer: Thank you very much for your valuable comments on this manuscript. We have revised Figures 5 and 6 in the manuscript.

10. L554: Please avoid using phrases such as "In Fig. 7" and instead directly present the main findings of the research. For example: "The effects of soil fungi on SOC priming significantly exceeded the effects of bacteria on SOC sequestration (R = 0.40 vs. 0.27, P = 0.003 vs. 0.02), leading to an increase in SOC mineralization and decomposition after the addition of exogenous C (Fig. 7)."

Answer: Thank you very much for your valuable comments on this manuscript. We have revised it in the manuscript. We've already removed "In Fig. 7, we present a conceptual diagram illustrating the main findings of this research."

11. Please consider citing the following paper: [https://doi.org/10.1016/j.scitotenv.2022. 158274] as it is relevant to the research topic.

Answer: Thank you very much for your valuable comments on this manuscript. We have added this reference to L58 of the manuscript.

Finally, it is pointed out in particular that the rigorous attitude and rigorous scientific thinking of experts have given us a lot of inspiration and help, so that we have further improved the understanding of article writing and scientific problems. Thanks again to the presiding experts and the editorial department 

Reviewer 2 Report

This paper is good, but some problems exist.

2.5. point is not well described. (line 226)

The instruments used are missing. Some references are missing too.

Fig 3-4. imige qulitiy is poor. These are must improve.

Table 3 The „days” unit is missing.

Line 277 “(Q_soi)” chage to “(Q_soi)”

Author Response

1. 5. point is not well described. (line 226). The instruments used are missing. Some references are missing too.

Answer: Thank you very much for your valuable comments on this manuscript. We added instruments and references. We have changed the “Soil pH was determined using the potentiometric method with a soil–water ratio of 1:2.5, and bulk density was measured using the ring-knife weighing method. The oil bath-heated potassium dichromate oxidation volumetric technique was applied to calculate the SOC content. Total nitrogen was determined by Kjeldahl distillation, while the molybdenum antimony colorimetric and the NaOH fusion–flame photometric methods allowed the estimation of total phosphorus and total potassium contents, respectively. The exchangeable Ca content was measured using the ammonium acetate exchange–atomic absorption spectrophotometry method. Soil enzyme activity was determined based on the methods described in Guan (1986). Soil urease, sucrase, and neutral phosphatase activity were determined using the phenol–sodium hypochlorite colorimetric method, the 3,5-dinitrosalicylic acid colorimetric method, and the sodium benzene phosphate colorimetric method, respectively.” to “Soil pH was determined using the potentiometric method with a soil–water ratio of 1:2.5 (pH meter, PHS-25), and bulk density was measured using the ring-knife weighing method. The oil bath-heated potassium dichromate oxidation volumetric technique was applied to calculate the SOC content (Thermostatic oil bath, HH-S). Total nitrogen was determined by Kjeldahl distillation (Automatic kieldahl apparatus, Kjeltec 8100), while the molybdenum antimony colorimetric and the NaOH fusion–flame photometric methods allowed the estimation of total phosphorus and total potassium contents (Ultraviolet spectrophotometer, UV-9000s and Flame spectrophotometer, FP6420), respectively. The exchangeable Ca content was measured using the ammonium acetate exchange–atomic absorption spectrophotometry method (atomic absorption spectrometer, ZEEnit -700P). Soil enzyme activity was determined based on the methods described in Guan (1986). Soil urease, sucrase, and neutral phosphatase activity were determined using the phenol–sodium hypochlorite colorimetric method, the 3,5-dinitrosalicylic acid colorimetric method, and the sodium benzene phosphate colorimetric method (Ultraviolet spectrophotometer, UV-9000s), respectively.”

2. Fig 3-4. imige qulitiy is poor. These are must improve.

Answer: Thank you very much for your valuable comments on this manuscript. We have re-adjusted Fig. 4 in the manuscript.

3. Table 3 The „days” unit is missing.

    Answer: Thank you very much for your valuable comments on this manuscript. We added “days” in Table 3.

4. Line 277 “(Q_soi)” change to “(Q_soi)”

Answer: Thank you very much for your valuable comments on this manuscript. We have changed the  “(Q_soi)” to “(Q_soi)”

Reviewer 3 Report

Line 87. Litter and calcium carbonate (CaCO3) … Can you explain more about which litter you are talking about?

Line 234-238. What was the purpose of determining the activity of soil enzymes?

Line 390. Fig.4 Characteristics of the soil microbial community composition at the genus level. The quality of the drawing is poor. The font is not readable.

Author Response

1. Line 87. Litter and calcium carbonate (CaCO₃) … Can you explain more about which litter you are talking about?

Answer: Thank you very much for your valuable comments on this manuscript.  The litter mentioned in our manuscript is mainly the material formed when the leaves, branches and stems of the vegetation fall on the soil surface in the forest ecosystem.

2. Line 234-238. What was the purpose of determining the activity of soil enzymes?

Answer: Thank you very much for your valuable comments on this manuscript. As an important index to characterize soil carbon cycle, soil enzymes directly participate in soil microbial activities, carbon decomposition and stabilization process. In our manuscript, soil enzymes are shown as the basic data of the tested soil.

3. Line 390. Fig.4 Characteristics of the soil microbial community composition at the genus level. The quality of the drawing is poor. The font is not readable.

    Answer: Thank you very much for your valuable comments on this manuscript. We have re-adjusted Fig. 4 in the manuscript.