Legacy Effect of Long-Term Elevated CO2 and Warming on Soil Properties Controls Soil Organic Matter Decomposition
Round 1
Reviewer 1 Report
Review of the manuscript: Legacy effect of long-term elevated CO2 and warming on soil properties controls soil organic matter decomposition
General comments: The manuscript is well written in English and reports the effect of plant litter quality and soil microbial community change on soil organic carbon mineralization under elevated conditions of atmospheric CO2 and temperature. The introduction, methods, results are well documented. However, there are some specific comments for the methods section that should be resolved before publication.
Comments to authors:
In the abstract section (page 1, line 6) said: liter, should say: litter
In page 2, third paragraph of the introduction said: content s, should say: contents
METHODS:
Please describe the soil type used in the experiment and the past soil use of the experimental field.
Also it would be nice to include a photo of the field experimental facility of the long term CO2 enrichment project.
RESULTS:
Regarding this section, It should be convenient to explain if the experimental data were parametric (normal distribution and homogeneity variance test).
Author Response
Reviewer 1：
Review of the manuscript: Legacy effect of long-term elevated CO2 and warming on soil properties controls soil organic matter decomposition
General comments: The manuscript is well written in English and reports the effect of plant litter quality and soil microbial community change on soil organic carbon mineralization under elevated conditions of atmospheric CO2 and temperature. The introduction, methods, results are well documented. However, there are some specific comments for the methods section that should be resolved before publication.
Comments to authors:
In the abstract section (page 1, line 6) said: liter, should say: litter
In page 2, third paragraph of the introduction said: content s, should say: contents
Responses: Accepted and revised.
METHODS:
Please describe the soil type used in the experiment and the past soil use of the experimental field.
Responses: Accepted and revised. The soil is a Gleyic Stagnic Anthrosol (WRB-FAO) derived from clayey lacustrine deposit and cultivated with summer rice-winter wheat rotation for hundreds of years. The basic properties of the topsoil before the experiment onset were: pH (H2O) 7.0, bulk density of 1.2 g cm−3, and contents of organic C and total N of 16.0 g kg−1 and 1.9 g kg−1, respectively.
Also it would be nice to include a photo of the field experimental facility of the long term CO2 enrichment project.
Responses: Accepted and revised. A photo of the experiment facility was provided in the revised manuscript (Fig. 1).
RESULTS:
Regarding this section, it should be convenient to explain if the experimental data were parametric (normal distribution and homogeneity variance test).
Responses: Yes, normal distribution and homogeneity were checked before One-way ANOVA analysis.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Dear colleagues! You have prepared an interesting and relevant paper. I have no doubt that it is suitable for Agronomy and will be published there soon. However, I have two comments that I ask you to correct.
The first concerns the methodology for calculating the emission rate of CO2 (equation at the end of page 4 of the manuscript). This equation is not clear, and most likely not quite correct. It is not clear that there is an alpha (conversion rate). The density of the gas cannot be constant, and it depends on temperature and atmospheric pressure. The correct calculation accepted in soil science can be found, for example, in doi: 10.1134/S1064229318010143 or in doi:10.3390/ma11101889 www.mdpi.com/journal/materials. It uses indicators of the volume content of CO2 directly measured on a chromatograph or gas analyzer, expressed in % or ppm (С% or Cppm), as well as data on temperature and atmospheric pressure. The conversion to the mass of the emitted CO2 is carried out according to the fundamental Clapeyron-Mendellev equation. The final formula looks like: F=(delta C%/delta t)*P*M*V/(100*R*T*m) if the device measures the volume content of CO2 in %, or F=(delta Cppm/delta t)*Xppm*P*M*V/(1000000*R*T*m) for Cppm, where P is the pressure in Pa, M is the molar mass in g/mol, R is the universal gas constant (8.314 J(mol K)), T is the absolute temperature in Kelvins. For example, if delta C% /delta t = 5% / day, the temperature of the experiment is 25 degrees C, the atmospheric pressure is 740 mm Hg or 98.6 kPa, the volume of air in the flask is 0.5 L (0.5/ 1000 m3), and the mass of absolutely dry soil m = 50 g (0.05 kg), the correct calculation will give F =5*98.6*1000*12*(0.5/1000)/(100*8.314*(273+25)*0.05)=0.239 g C-CO2/(kg day) or 239 mg of C-CO2/(kg day). If the atmospheric pressure on that day was normal (760 mm or 101.3 kPa), then with the same increase in the volume content of CO2 (5%/day) we will get F=245 mg C-CO2 / (kg day), that is 6 mg C more. This example shows the need to take into account not only the temperature of the experiment, but also atmospheric pressure. For the future (not for this article) I also strongly advise you to take into account the interphase interactions of CO2 with the solid (sorption) and liquid (dissolution) phases of the soil, which has a serious impact on the assessment of the CO2 emission rate in incubation experiments, as shown, for example, by doi: 10.1134/S1064229318010143.
The second remark concerns Figure 1. Please explain in the caption that there are insets. Apparently, this is also the release of CO2 in case of flooding of the soil, but then explain it ( insets show ....), and also indicate the names of the axes on the first inset. For the future (not for this article) I recommend that you monitor the redox potential or dissolved O2 in order to confidently talk about anaerobiosis. Because with a large volume of oxygen in the bottle, even flooded soil can have a sufficient amount of O2 in the soil solution essential for aerobic microbes.
I am waiting for your paper after corrections and hope that it will be published in Agronomy soon.
Feb.17.2023
Best regards, Your Reviewer
Author Response
Reviewer 2：
Comments and Suggestions for Authors
Dear colleagues! You have prepared an interesting and relevant paper. I have no doubt that it is suitable for Agronomy and will be published there soon. However, I have two comments that I ask you to correct.
The first concerns the methodology for calculating the emission rate of CO2 (equation at the end of page 4 of the manuscript). This equation is not clear, and most likely not quite correct. It is not clear that there is an alpha (conversion rate). The density of the gas cannot be constant, and it depends on temperature and atmospheric pressure. The correct calculation accepted in soil science can be found, for example, in doi: 10.1134/S1064229318010143 or in doi: 10.3390/ma11101889 www.mdpi.com/journal/materials. It uses indicators of the volume content of CO2 directly measured on a chromatograph or gas analyzer, expressed in % or ppm (С% or Cppm), as well as data on temperature and atmospheric pressure. The conversion to the mass of the emitted CO2 is carried out according to the fundamental Clapeyron-Mendellev equation. The final formula looks like: F=(delta C%/delta t)*P*M*V/(100*R*T*m) if the device measures the volume content of CO2 in %, or F=(delta Cppm/delta t)*Xppm*P*M*V/(1000000*R*T*m) for Cppm, where P is the pressure in Pa, M is the molar mass in g/mol, R is the universal gas constant (8.314 J(mol K)), T is the absolute temperature in Kelvins. For example, if delta C% /delta t = 5% / day, the temperature of the experiment is 25 degrees C, the atmospheric pressure is 740 mm Hg or 98.6 kPa, the volume of air in the flask is 0.5 L (0.5/ 1000 m3), and the mass of absolutely dry soil m = 50 g (0.05 kg), the correct calculation will give F =5*98.6*1000*12*(0.5/1000)/(100*8.314*(273+25)*0.05)=0.239 g C-CO2/(kg day) or 239 mg of C-CO2/(kg day). If the atmospheric pressure on that day was normal (760 mm or 101.3 kPa), then with the same increase in the volume content of CO2 (5%/day) we will get F=245 mg C-CO2 / (kg day), that is 6 mg C more. This example shows the need to take into account not only the temperature of the experiment, but also atmospheric pressure. For the future (not for this article) I also strongly advise you to take into account the interphase interactions of CO2 with the solid (sorption) and liquid (dissolution) phases of the soil, which has a serious impact on the assessment of the CO2 emission rate in incubation experiments, as shown, for example, by doi: 10.1134/S1064229318010143.
Response: Thank you for your suggestion. The reason why our formula is different from yours is that we use the density of CO2 under standard atmospheric pressure, so the formula is simplified. Our formula is referred to Lu et al. (2014) and Hu et al. (2004). It's definitely better to use the actual pressure, but a lot of the references use the standard pressure, and we use the standard pressure for comparison with other studies. In addition, we added the conversion coefficient and corrected the ρ unit in the revised manuscript.
Hu, R.; Hatano, R.; Kusa, K.; Sawamoto, T. Soil Respiration and Net Ecosystem Production in an Onion Field in Central Hokkaido, Japan. Soil Science and Plant Nutrition, 2004, 50, 27–33, doi:10.1080/00380768.2004.10408449.
Lu, W.; Ding, W.; Zhang, J.; Li, Y.; Luo, J.; Bolan, N.; Xie, Z. Biochar Suppressed the Decomposition of Organic Carbon in a Cultivated Sandy Loam Soil: A Negative Priming Effect. Soil Biology and Biochemistry, 2014, 76, 12–21, doi:10.1016/j.soilbio.2014.04.029.
The second remark concerns Figure 1. Please explain in the caption that there are insets. Apparently, this is also the release of CO2 in case of flooding of the soil, but then explain it (insets show ....), and also indicate the names of the axes on the first inset. For the future (not for this article), I recommend that you monitor the redox potential or dissolved O2 in order to confidently talk about anaerobiosis. Because with a large volume of oxygen in the bottle, even flooded soil can have a sufficient amount of O2 in the soil solution essential for aerobic microbes.
Response: Thanks for your valuable comments. We added an explanation about the inset. “The inset represents the period of flooded incubation from Day 65 to Day 147.” We will consider the measurement of soil redox potential during incubation as it can reflect the O2 status in the bottle, which has a significant impact on CO2 emission.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Dear colleagues, you did not consider it necessary to correct the formula and calculations of the CO2 emission rate for more correct ones, although I showed you in the example what errors occur if you neglect the dynamics of atmospheric pressure. Your choice reduces the quality of the paper, but I will not enter into into further controversy with you and agree that your paper can be published in its current form.
03.03.23
Best wishes, Reviewer
