Methane emissions in triple rice cropping: patterns and a method for reduction

We hope the misunderstandings should be clarified in the response so that we could know the misunderstandings. You didn't give us any response so we understood that you accepted our response. You and we should reduce the effort of the Editorial Team.

Thank you for reviewing and we are sorry to late to respond.

1) For example, the authors cited their own articles, and readers cannot clearly distinguish ones from the previous studies and one of this study. 

Oda & Chiem (2019) is a field experiment conducted in the year 2018; instead, this work is "a specific analysis of a part of the data set; Introduction L. 15" of an old project (2011-2016). We are sorry that the relation is a little complicated; however, that is a fact. What we call "data analysis" can do at any time.

We welcome if you give us a piece of advice for the way to dissolve this problem.

2) There is some inconsistency between the previous study of Oda and Chiem (2019) — “the absence of rice plants doubles the methane emission from the field because of the lack of methanogenesis inhibition by rice plants” — and the Figure 1 of this study. 
The authors are not attempting to discuss this inconsistency but try to discuss their new findings without citing any supporting evidence from other previous works.

The sentence is in the result section and the inconsistency is discussed in the discussion sections as follows. 

"This can be explained by the accumulation of rice residue from the preceding crops, especially by the rice straw incorporated into the soil, because the contribution of the present rice-derived carbon is small; Discussion L. 2"

Remember that "Just after the 3rd crop, the natural flood starts so the straw is left on the paddies; Introduction L. 10". No rice straw is incorporated before the flood period. No rice straw is incorporated before the flood period. That is the difference in the conditions of Oda & Chiem 2019.


We are waiting that your response.

Thank you for deeply considering our manuscript. We considering your helpful comments and improved the manuscript. However, we are afraid that adopting the term "fallow flood" is not right in case of the Mekong Delta triple cropping rice. In addition, we think the matter of writing style is not constructive because that is largely affected by the background of the main journal we have read. Finally, we are glad to have helpful comments from you. The details of our response are as follows.

Introduction:
•    Background (the need for this study) and objectives of the study should be clearly presented. Why did you study methane emissions from paddy fields? What gaps of knowledge did you try to approach? What are the objectives of this study?
*As you mention "there are very few studies that reported methane emissions from triple rice cropping." That is the reason. We described more clearly the aim of the study.
 
Methods:
•    The amount of rice straw 
*They were added.

Data analysis:
•    No statistical analysis was made for the data in Figure 1.
*You mention the significant tests, don’t you? Now, that is no longer recommended; instead, 95% confidential interval is recommended (https://www.nature.com/articles/d41586-019-00857-9). That can be known from the SD and the n. However, we found that the figure is needed correction of the way of calculation of the mean (each crop has three replications but means should be calculated by year, and the n should be changed from 15 to 5). In addition, the error bars were not SD but were SE. We corrected the figure and show the 95% confidential interval (CI). We deeply thank that you give us a chance to correct the error bars.

Results:
•    In this section, the results of experiment should be simply presented. Information of previous studies and discussion with them should be presented in Introduction or Discussion sections, respectively. From this, the sentences in Line 1–6 of ‘Emission level’ part and in Line 1–11 of ‘Emission pattern’ part should be moved.
*This is a matter of writing style. The consideration of the values using citations is common in the results section.
 
Data presentation:
•    The amounts of total seasonal methane emission should be numerically presented, at least those of five-year average for each crop with the values of interannual variation.
*We showed them as a range, but we changed that as each average.

Discussion:
•    There is fundamental lack of discussion with referring to previous studies by other researches who reported methane emissions from paddy fields. A sweeping revision is necessary in this section.
*This is a matter of writing style. We cited the studies in the introduction section. As you know, they are few.
 
In addition, some minor comments are listed below:
1.    Throughout the manuscript, the term ‘natural flood’ or ‘annual flood’ should be corrected to ‘fallow flood’ because it is confusing. ‘After natural (annual) flood’ can be ‘during the fallow flood period after harvesting the 3rd crop’.
*There is no fallow flood. We show the condition by describing the cropping days and the intervals in the manuscript.
 
2.    Abstract, Line 2: The statement that ‘these emissions are not well studied’ is incorrect. A number of field measurements for methane emissions from paddy fields in the Mekong Delta have been published, such as Vo et al., 2018 (already cited in this paper), and references cited in the paper.
*We cited the description “not well studied” of Vo et al. The situation has not changed after a year.
 
3.    Introduction, 1st paragraph, Line 9: The statement that ‘this has not been well studied’ is incorrect. IPCC Guidelines provides the quantitative effects of rice straw incorporation on increasing methane emissions as a scaling factor based on a number of field measurements including those in the Mekong Delta. 
*See above. We think this kind of rebuttal is not constructive.
 
4.    Introduction, 3rd paragraph, Lines 6–8: The reference that shows effectiveness of the strategy should be cited.
*This is a matter of writing style. Showing the main finding of the work at the end of the introduction section is common.
 
5.    Methods, Site description: It is suggested to report the type of soil and its characteristics, because the information is essential for discussing the intensity of methane emissions from paddy fields and is not presented in the previous paper. I would suggest to report, at least, soil texture, organic carbon content, and pH, all of which are recognizes as the major factors controlling methane emissions. 
*The details of the soil properties will be published in another paper. That is the reason why we don’t describe the details. We think this kind of problem that comes from closed science should be diminished in the future.
 
6.    Figures 1 and 2: The period of data ‘Flood’ is confusing. It is recommended to correct it with ‘Fallow flood’.
*We think the word “Fallow flood” imagines a fallow paddy with ponding water. The paddy in the Mekong Delta is really a flood, a natural disaster.
 
7.    Results, Emission level, Lines 9–11: The emission level doubled in the 2nd crop, and tripled in the 3rd crop, compared with that in the 1st crop, then reset after the fallow flood. Same correction should be made at Discussion, Emission pattern, Lines 1–3.
*You understood correctly without the phrase, “compared with that in the 1st crop”. Flood is not fallow. We think this kind of rebuttal is not constructive.
 
8.    Results, Emission level, Lines 13–: The total emission during the fallow flood should be higher …
*We cannot agree to use the term.
 
9.    Results, Emission level, Lines 13–15: Why the result was not consistent to the ‘previous’ report. Please discuss it.
*That is in the discussions section. The reset of emission levels after the annual flood means that the rice straw is decomposed without methanogenesis in water because the water includes dissolved oxygen.
 
10.    Results, Emission pattern, Lines 1–11: This statement is not the results in this study. Therefore, it should be moved to Discussion. Also, the difference of data in ‘previous’ study and this one should be clarified.
*Generally, “what the data are” is describes in the results section; instead “what the data meaning” is describes in the discussions sections. According to these criteria, Lines 1-11 is suitable for the results section. 

Understanding from the ‘previous’ paper (Oda & Chiem, 2019), the ‘previous’ study reported the results of experiments during 2016 and 2017, whereas this study during 2011 and 2016. If it is correct, using the term ‘previous’ is not appropriate.
*Only published studies can cite in the paper. They are called previous studies, not future studies. We think this kind of rebuttal is not constructive.
 
11.    Discussion, 2nd paragraph, Line 6: no methane emission -> little methane emission
*Theoretically and practically "no" but we follow your recommendation for avoiding unconstructive rebuttal. Thank you.

1) Methodology
We followed the standard method so replication by others is capable. As you know,  to say overestimate requires grounds; however, data for methane emission in triple cropping rice is little. Note conditions of the reference we indicate are milder than that of the present study. We think the discussion should start waiting for more data.

Importantly, the theme of this brief report is not the emission levels but is the pattern, especially the role of the flood period reducing methane emission. They are not affected by whether we overestimated or underestimated. 

2) Discussions
We guess that you might be confusing the personal response and the manuscript because of the system. That is actually "out of the ring".

We hope you agree with the above explanations.

We found an example of high emission in Japan (https://www.naro.affrc.go.jp/org/warc/research_results/skk_seika/h06/94017-z1.jpg). The total amount roughly estimated about 2000 kg CH4 ha-1 crop-1. The result obtained under the condition of 5 Mg of wheat straw and 6 Mg of rice straw. Wheat straw is rapidly decomposed than rice straw. The fresh rice straw in the Mekon Delta triple-cropping rice is considered the middle of those organic materials. We added the information of the quantity of applied rice straw to the revised manuscript. That is about 9 Mg per crop. We think this additional information provides enough reason to understand the high emission in our results.

Thank you very much for giving valuable comments.
 
1. Introduction: a general information in order to explain this study is a representative
Thank you. We added.
 
2. Methods
2-1. Site description:
The sentence was improved to clarify that the present study was an observation of typical farmer’s paddies mentioned in the introduction (including rice straw treatment).
 
2-2. Farmers practices:
Thank you. We added the reference. The height of the stubble is about 30 cm. The daily average water levels were monitored with water level loggers at the corner of the fields; the average levels were 2.0 cm (−0.6 to 6.1 cm) until drained (the data will be published in another paper).
 
2-3. Methane measurement:
Thank you. We added.
 
3. Statistical analysis:
Figure 1 is clear enough to omit ANOVA (see the standard deviation (not SE)).
 
4. Discussion:
The information on straw incorporation in the observed fields is mentioned in the introduction. Rice straw incorporation enhances methane emission. We added a referred to our previous study that is a kind of short review of the effect of straw incorporation.