Responses of Soil CO2 Emission and Tree Productivity to Nitrogen and Phosphorus Additions in a Nitrogen-Rich Subtropical Chinese Fir Plantation
Round 1
Reviewer 1 Report
References – Very Improper referencing!!! It can affect not only an individual article’s citation index but also such erroneous and inaccurate quotations, if continually repeated, may turn inaccuracies into conventional knowledge.
When working on a reference list, it is important to cite the correct references and provide complete and accurate information for each publication
Lines 199-202, 219-220, 224-225 - all statistical symbols must be italicized
see comments in the text
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Reviewer 2 Report
MBC and MBN parameters are discussed, however, no results are shown in the manuscript (and no method of MBC and MBN measurement is described).
L323_326: After N addition, most of the added N will leave the ecosystem soon or remain in the soil in the form of organic N, and can not be absorbed and utilized by plants and microorganisms, which cause non-significant effects on CO2 emission
What is the mechanism of transformation of inorganic N into organic matter?
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Reviewer 3 Report
The study investigated how N and P additions affected soil CO2 emissions and tree productivity in an N-rich Chinese fir plantation (Cunninghamia lanceolata), and further explored the underlying mechanisms. Results showed that N addition had no effect on soil CO2 emissions and tree productivity, and P additions could increase tree productivity and soil CO2 emissions at short time period. These results could help provide guides on sustainable carbon and productivity management for Chinese fir plantations in the subtropical region. I still have some comments and suggestions.
1. Give more detail description about soil sampling (for example sampling time, duplicates`` ) and soil physico-chemical properties. More soil data such as soil pH, soil available P should be given. This paper introduced that the fir forest is N-saturated or no N-limitation, is there any direct evidence which certify that the studies fir forest is N-saturated?
2. As introduced by line 171, the first soil CO2 emissions sampling was conducted on March 20, 2019 (before treaments). Soil CO2 emissions at the first sampling time seem significantly different among different sites (as showed in Figure 5). This meant that the basal soil CO2 emission rates varied significantly before N and P addition. Thus, the significant variation in soil CO2 emissions after N and P addition might also cause by or partly cause by their locations. Try to remove the effect of basal variations.
Besides the basal variation in soil CO2 emissions, soil physico-chemical properties might also vary significantly among different site locations before N and P addition. If the authors have these soil data before N and P addition, try to remove the effects of basal variations.
3. In this study, N and P addition were only carried out at one 20*20 plot, and four subplots were used to determine soil CO2 emissions. Thus, the duplicates seem a Pseudo replication. When calculation the tree productivity in figure 6, the value was calculated at the individual level, but not plot level. The credibility of the results should be re-considered.
4. Figure 4, the data of the insert graph in figure 4 were the mean value of the all months or the first four month? Please noted in the caption. Moreover, the lowercase letters near the error bar were different with the data showed in table 1. Please check it.
The x-axis in Figure 4 was data of each sampling time. If the author wanted to show the monthly dynamics of soil CO2 emissions, the axis scale interval should remain consistent (note: 2019-5 and 2020-5). Also, the unit of soil CO2 emissions should kept consistent (e.g. mg·CO2/m2 ·h, mg /m2 ·h, mg /(m2 ·h)), please check the whole manuscript.
5. Figure 5, the data used in this the model were the mean value of the all months or the first four month or all measured data? And also, soil data were from which sampling time? Please described clearly in the material and method part.
The purpose of SEM analysis was to invest the effect of N and P additions on soil CO2 emissions. Did the N and P additions affect SM and ST, did the SEM model try to explain the month pattern of soil CO2 emissions? if not, SM and ST need not exist in the model. Moreover, N and P additions could affect tree productivity, which than regulate soil WSOC, MBC and other parameters (as explained in the discussion part). Thus, tree productivity can exist in the model. SEM Model structure needs to be adjusted.
6. The statistical analysis showed N and P had no significant interaction effect on soil CO2 emissions. Why paragraph from line 353 try to explain that their existed a N and P interaction?? Ine 361, the data didn’t show that “LP addition had higher soil CO2 emissions than HNLP”.
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Round 2
Reviewer 2 Report
The manuscript either brings no new information, or offers nonsenses.
CO2 production is sometimes increasing, sometimes decreasing, sometimes is the same... You do not offer relevant explanations of your findings. The statements are sometimes contradictory
21-22: The results indicated that soil temperature and moisture significantly correlated with the soil CO2 emissions.
This is no surprise since the decomposition of organic matter depends on temperature and moisture
95: Subtropical forest soil is the second largest source of atmospheric CO2 after tropical forest.
Does it mean that those forests are sources only, not sinks?
141: These plots are over 10m away from the major roads and other ecosystems. 10 meters from main road seems to be relatively close to additional source of reactive nitrogen.
258-262: This implied that the N and P interaction may greatly affect the 258 soil carbon dynamics. On the annual time scale, the HN, HNLP, HNHP and HP treatments increased soil CO2 emission by 6.71%, 20.26%, 26.58%, 29.98% and 16.24%, respectively compared with the CK. However, the difference was not significant (P>0.05; Table 1), which is because of the high internal variations of CO2 emissions among all replicates under each treatment.
Numbers with too many decimal places (btw. a problematic issue throughout the whole manuscript) showing that the difference is insignificant. Why this is there?
284-285: The addition of phosphorus significantly increased WSOC and further increased MBC, resulting in a significant increase in CO2 flux. Phosphorus addition suppressed CO2 emissions by reducing MBN.
This statement is contradictory. Are CO2 emissions suppressed, or increased by P addition?
285: Phosphorus addition suppressed CO2 285 emissions by reducing MBN. How could P addition decrease N in microbial biomass? Seems like nonsense.
282-283: Nitrogen addition significantly and positively affects NO3--N, while NO3--N indirectly promotes CO2 emissions by increasing NH4+-N. How NO3 increases NH4? How can NH4 increase CO2 emissions?
286-287: Soil temperature and humidity promote CO2 emission by increasing soil NH4+-N. What? Increasing NH4-N increases CO2 emissions? Are you sure about that mechanism? Are you able to describe it?
L345-346: "Our study also proved that the MBC significantly increased by 115% and 148% after LP and HP treatments, respectively." Here the authors discuss results - more than twice increased values of C in microbial biomass - but those results are not shown in the manuscript
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Reviewer 3 Report
The paper has been improved, but I still have some comments and suggestions.
1. Although the author said their previous studies have described some data ((including NO3--N, NH4+-N, MBC, MBN, WSOC—these data were used in the SEM model), they did not mention these data in this study to avoid repeating. But I still think the author should introduced these data (describe the data in a simple way, and data table/figure as supplementary material). These data were important for understanding the SEM model.
2. Still about the SEM model, the author said CO2 flux data were from all measured month. Data of tree productivity was only detected for three times (pretreatment, right at the end of the experiment and three years after the experiment), thus tree productivity can’t be included in the SEM model. My question was that the author has data of soil properties (NO3--N, NH4+-N, MBC, MBN, WSOC) for each month?? But in the method, soil and leaf were sampled for four times (on March 19, June 20, September 20, 2019 and May 20, 2020. How did these soil data match the CO2 flux data??
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