F1000Research F1000Research 2046-1402 F1000 Research Limited London, UK 10.12688/f1000research.110326.2 Brief Report Articles Rice straw decomposition in paddy surface water potentially reduces soil methane (CH 4) emission

[version 2; peer review: 3 approved with reservations, 1 not approved]

 Van Thao Huynh Conceptualization Data Curation Formal Analysis Methodology Writing – Original Draft Preparation https://orcid.org/0000-0001-6582-9061 1 Oda Masato Conceptualization Methodology Supervision Validation Writing – Review & Editing https://orcid.org/0000-0001-7241-5238 2 Chiem Nguyen Huu Conceptualization Methodology Supervision Writing – Review & Editing a 1 College of Environment and Natural Resources, Can Tho University, Can Tho city, 900000, Vietnam Japan International Research Center for Agricultural Sciences, Tsukuba city, 305-8686, Japan nhchiem@ctu.edu.vn

No competing interests were disclosed.

 11 7 2022 2022 11 298 17 6 2022 Copyright: © 2022 Van Thao H et al. 2022 This is an open access article distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Background: Rice cultivation is considered significant methane (CH 4) emission source. Rice straw (RS) incorporation into the soil is a critical factor that produces higher CH 4 emissions. The RS waterlogging approach on the soil surface potentially reduces CH 4 emissions due to increasing RS decomposition in aerobic conditions. However, the hypothesis has not been entirely determined. The study aimed to examine CH 4 emissions under RS waterlogging on surface water compared with RS incorporation into the soil.

Methods: We carried out a microcosm experiment in a screen-house with two treatments, including (i) RS incorporation into the soil and (ii) RS waterlogging on the soil surface in triplicates. We compared the CH 4 emissions and CH 4 accumulation for the off-growing and rice-sowing periods. Yield-scaled CH 4 emission was assessed based on total methane emission and rice yield.

Results: The results demonstrated that RS waterlogging reduced CH 4 emissions by 16.9% compared to RS incorporation. During the rice-growing period, total CH 4 emissions from RS waterlogging accounted for 36% of the incorporation treatment. Under RS waterlogging, high CH 4 emissions during the off-sowing stage were disclosed. The difference between yield-scaled CH 4 emissions was not significant.

Conclusions: RS waterlogging is a feasible option to alternate conventional RS incorporation toward lower CH 4 emissions from rice production. Ameliorating CH 4 emission mitigation by RS waterlogging during off-sowing is recommended for future works.

greenhouse gas emission rice straw incorporation methane emission organic rice water management the Vietnamese Mekong Delta The author(s) declared that no grants were involved in supporting this work. Amendments from Version 1

We have incorporated the reviewers’ suggestions and recommendations in the new version. Accordingly, the revisions have been made as follows:

The conclusion has been revised to be clearer in the Abstract section.

The significance and objectives have been revised in the Introduction section.

The Methods section has been rearranged and provides more information.

Values in Figures have been added in the Results section.

An additional explanation has been expanded related to CH 4 emission in the Discussion section.

Discussion related to yield and rice straw has been added.

Space and units missing have been unified throughout the paper.

We added data in Figure 1 (above each column) as required by Reviewer 1. There was no other revision.

We added data in Figure 2 (above each column) and added Figure 2a as required by Reviewer 1. The title of Figure 2 has been expanded. There was no other revision.

We added the off-sowing and rice-growing periods in Figure 3 as required by Reviewer 1. There was no other revision.

Introduction

The agricultural sector contributes approximately 10–12% of global anthropogenic emissions. Of these emissions, 47% of methane (CH 4) emissions have been attributed to agriculture production ( Smith et al., 2008). Rice fields have been considered an important source of atmospheric CH 4, accounting for 15–20% of the global total anthropogenic CH 4 emissions from agriculture ( Phung et al., 2021; Sass & Fisher, 1997). Rice straw (RS) plays a vital role in contributing to CH 4 emissions from paddy fields. Several field experiments have shown that RS incorporation significantly increases CH 4 emissions in rice fields ( Hoang et al., 2019; Jiang et al., 2019; Liu et al., 2015; Wang et al., 2019). In the Vietnamese Mekong Delta (VMD), intensive rice cultivation in combination with RS incorporation treatment significantly contributes to CH 4 emissions ( Oda & Nguyen, 2019). Thus, developing RS treatment practices to gain lower CH 4 emissions has generated considerable recent research interest. Recently, several studies suggested that decomposing RS in paddy surface water could be a practical technique for reducing CH 4 emissions ( Boateng et al., 2017; Oda & Nguyen, 2019; Tariq et al., 2017). However, the timing of RS waterlogging before sowing has not been recommended. In a field experiment, Thao et al. (2019) reported that a 20-day period of RS decomposition in surface water (off-sowing) was suitable for growing rice in the VMD's double-cropping pattern. However, the research did not reveal CH 4 emissions from the treatment. In addition, Oda & Nguyen (2019) suggested that the strategy of decomposing RS on surface water could effectively reduce CH 4 emissions from the paddies. To the best of our knowledge, CH 4 emissions from the RS decomposition process in paddy surface water have not been thoroughly studied. Therefore, we aimed to elucidate CH 4 emissions by the RS waterlogging method compared with RS incorporation into the soil during the off-sowing and rice-growing period, referred to as a conventional rice cultivation practice in the VMD.

 Methods Experimental site and materials

This study was conducted in an experimental screen house at Can Tho University (Cantho City, Vietnam) from April to August 2019. The screen house consisted of a translucent white roof and rat-proof wire screens on the side. The inner humidity in the screen house was relatively similar to that outdoors.

We used plastic containers (38 cm in length × 58 cm in width × 30 cm in height) to carry out the microcosm experiment. Each container was filled with paddy soil up to 20 cm. The soil was collected from a farmer's field in Cantho city (10°18′N, 105°54′E). The soil was classified as Thionic Glycesol ( Dong et al., 2012). The rice cultivar used for the experiment was a short-duration variety (IR50404, 85–90 days) provided by the Cuu Long Delta Rice Research Center. The IR50404 cultivar is popularly used in the VMD.

 Experimental design

The examination of RS waterlogging and RS incorporation into the soil on CH 4 emissions were followed by a conventional rice cultivation experiment (first crop: Jan-6 to Mar-31). The first experiment was conducted to set essential conditions (rice straw, soil) for implementing the second experiment. In the first crop, pre-germinated seeds were sown at an equivalent rate of 250 kg ha -1 on wet-levelled soil. The seeding rate was based on common application by the majority of farmers in VMD. Water irrigation was managed as alternative wetting and drying (AWD) technology which reflood 5 cm when the surface water level naturally declined to 10 cm below the soil surface. The technology is known as multiple aerations, developed by International Rice Research Institute (IRRI) to reduce water consumption for rice cultivation ( Tran et al., 2018). In the second crop, we used RS and soil from each container in the first crop correspondingly to experiment with the effects of RS waterlogging and RS incorporation on CH 4 emissions.

There were two treatments, comprising (i) RS waterlogging on the soil surface and (ii) RS incorporation into the soil. Each treatment was set up in triplicate. Fresh RS (above-surface biomass) in the first crop was collected and cut into 5 cm in length. In the waterlogging treatment, RS was scattered on the soil surface and irrigated to 10 cm in-depth. Then, the RS was gently pressed into water. It was left for a 20-day stage without disturbing (off-sowing period). The timing was followed by a field demonstration recommended by Thao et al. (2019) that RS was well-fermented in the surface water within 20 days, which was suitable conditions for broadcasting rice seeds. In the RS incorporation treatment, RS was well incorporated into the soil by a shovel. The soil depth was mixed at approximately 20 cm in depth. Then, it was immediately irrigated to 2 cm in-depth for a 5-day period (off-sowing period). RS incorporation into the soil treated for 5 days was a typical treatment pattern for a triple-cropping rice production system in the VMD. The amount of RS applied for all treatments was the exact amount collected in the container of the first crop, correspondingly. On average, applied RS for the treatments of waterlogging and incorporation was 0.72 kg (dry weight) m -2 and 0.59 kg (dry weight) m -2, respectively. The term off-sowing used throughout the paper indicates a period that RS treated before sowing, which was 20 days for the RS waterlogging treatment and 5 days for the RS incorporation treatment.

A total of six microcosms were laid out closely in an array of the screen house with two columns and three rows. RS treatments were performed one day after harvesting the first crop. On the sowing day, we drained the field and leveled it by hand. The soil was not reincorporated. Pregerminated seeds of the IR50404 variety were also sown at a rate equivalent to 250 kg ha -1. Water irrigation was managed as a continuously flooding management method during the rice-growing period. Tap water was directly irrigated for each rice container. We started irrigation on day 7 with 3–5 cm of water and maintained the water level during the rice-growing period. The water level was drained seven days before harvesting. Fertilizers were not applied for either experiment because fertilization could change the CH 4 emission patterns.

 Measurements

The closed chamber method was used to measure CH 4 following the guidelines recommended by Minamikawa et al. (2015). The chamber (58 cm in length × 38 cm in wide and 90 cm in height) was equipped with a vent to allow equilibration of the pressure, a thermometer, a sampling port, and a fan to ensure well-mixed air inside the chamber while taking the gas sample. Gas sampling was flushed five times with chamber air before collecting. Gas samples were collected with a propylene syringe 50 mL at 3 and 23 mins after the chamber placement, and each gas sample was immediately injected to 15 mL in vacuumed vials. During the off-sowing period, gas sampling was taken on days 3, 6, and 13 for the rice straw waterlogging treatment, while RS incorporation treatment was sampled on day 3. The difference in the number of gas sampling times during off-sowing was due to a more prolonged RS treatment in the water logging period. After sowing, sampling frequently intensified every three days during the first 21 days when the high fluxes were characteristically observed. Then, the process was carried out once a week until the day of harvest. All gas samples were taken between 07:00, and 10:00 am. The CH 4 concentration was analyzed by gas chromatography (Shimadzu GC2014, Japan) equipped with a flame ionization detector, using 60/80 Carboxen® 1000 column at the temperature of 180 °C. Nitrogen (99.99%) as a carrier gas at a flow rate of 30 mL min −1.

Water levels were checked by a 50-cm ruler (1-mm scale). Grain yield was detected by harvesting all rice in each pot at physiological maturity and removing all unfilled grains using tap water. Then it was sundried at ambient temperature. The presented grain yield was adjusted to 14% of moisture by a grain moisture tester (Riceter f2, Kett Electric Laboratory, Tokyo, Japan).

 Data processing

The cumulative CH 4 emissions were calculated using a trapezoidal integration method with linear interpolation and numerical integration between sampling times. The calculation was done as follows: (i) calculate the daily gas flux by multiplying the daily mean hourly gas flux by 24, (ii) calculate the emission between every two consecutive measurements using the trapezoidal rule, and (iii) sum up the areas of all the trapezoids. Detailed guidance can be found at Minamikawa et al. (2015). Yield-scale CH 4 emissions was calculated by dividing total CH 4 emissions by grain yield. All measurements were carried out with three repetitions. Data processes were performed using Microsoft Excel 2019.

 Statistical analysis

Data analysis was performed using IBM SPSS Statistics 22.0 (RRID:SCR_016479). The independent sample t-test comparison was used to compare the CH 4 emission in rice-growing and off-sowing periods, rice yield and yield-scaled CH 4 emissions between treatments. The statistical significance was done with a confident level of 95%.

 Results Accumulative emissions

We assessed the effects of rice straw management via waterlogging and incorporation on CH 4 emissions using a microcosm experiment. The results showed that the CH 4 emission from waterlogging accounted for 36% of the incorporation treatment during the rice-growing period ( Figure 1a) ( Oda et al., 2020). However, high emissions were found in the off-sowing stage of the treatment by waterlogging ( Figure 1b). The total CH 4 emissions from the waterlogging and incorporation treatments were 502 ± 111.4 kg CH 4 ha -1 crop -1 and 604 ± 41.9 kg CH 4 ha -1 crop -1, respectively. In general, the magnitude of seasonal CH 4 emissions observed in our study was lower than what was found in previous studies on triple-cropping in VMD, which ranged between 710 and 1,789 kg CH 4 ha -1 crop -1 ( Oda & Nguyen, 2019; Vo et al., 2018). The CH 4 emission for the decomposing RS subject to waterlogging was 16.9% lower than that of the straw in the incorporation approach, even though the total timing of the off-sowing and rice-growing period was 30% longer. The yield recorded between treatments was no significant difference (P>0.05) ( Figure 2a). For the yield-scaled CH 4 emissions, the waterlogging and incorporation treatment were 0.21 ± 0.02 kg CH 4 kg-grain -1 and 0.30 ± 0.08 kg CH 4 kg-grain -1, respectively. The difference between yield-scaled CH 4 emissions was insignificant (P>0.05) ( Figure 2b).

CH 4 emissions accumulation of rice strawy (RS) waterlogging and RS incorporation in the periods of rice-growing ( a), and off-sowing ( b).

Grain yields ( a) and yield-scaled CH 4 emissions ( b) of RS waterlogging and RS incorporation.

 Emission pattern

CH 4 emissions peaked one week after the prior crop's harvest. The peak of waterlogging was 3.82 times higher than the peak of the incorporation approach ( Figure 3). After sowing, the CH 4 emission of the RS incorporation approach was always higher than that of waterlogging treatment. The peak was in line with a previous study ( Oda & Nguyen, 2019). The emission patterns displayed a gradual reducing tendency to the end of the rice-growing period.

CH <sub>4</sub> emission rate of rice straw (RS) waterlogging and RS incorporation. Discussion

Conventional rice cultivation based on RS incorporation of paddy fields is a substantial source of CH 4 emissions. Modifying RS practices is undoubtedly necessary to reduce CH 4 flux when the RS is incorporated into the soil. Although RS amendment enriches soil organic carbon and improves soil fertility ( Liu et al., 2014; Ole Sander et al., 2014), it increases organic carbon availability and simultaneously intensifies strict anaerobic conditions to stimulate CH 4 production in rice field ecosystems ( Sass et al., 1991). The strategy of RS waterlogging instead of incorporation demonstrated less CH 4 emissions by 12%, even though RS applied during the fallow period decreased CH 4 emissions by 11% compared to the same amount of RS applied during rice field preparation ( Lu et al., 2000). In this study, we reached similar results in mitigation of emissions from the RS practice with regards to waterlogging treatment by 16.9% reduction. The effectiveness of CH 4 reduction was achieved whenever (i) the amount of RS applied in waterlogging treatment (0.72 kg m -2) was slightly more than the RS applied in the incorporation treatment (0.59 kg m -2) and (ii) the prolongation of off-sowing and rice-growing periods which was 20 days and 5 days, respectively. The efficiency of CH 4 emission mitigation from RS waterlogging on the field surface is more likely attributed to the promotion of RS decomposition in an aerobic environment. Although the decomposing RS under waterlogging offered the efficaciousness of CH 4 emission diminution, non-fertilization could reduce the rice yield compared with the previous reports in the VMD ( Nam et al., 2022; Uno et al., 2021).

This study exhibited the effects of RS management practices on CH 4 emission in the rice-growing period. Total CH 4 emissions of RS after waterlogging were lower than that of the incorporation approach. These findings suggest that when RS was decomposed in water generated less CH 4 emissions than when it was buried in the soil. This could be partly explained by RS decomposition under flooded soil blocking oxygen penetration processes into the soil. The shortage of oxygen could create a stable anaerobic condition, allowing bacteria capable of producing CH 4 to thrive ( Conrad, 2007). In contrast, decomposition of RS in water is generally affected by dissolved oxygen; methanogenesis fermentation can be limited by high O 2 concentrations ( Jiang et al., 2019). In addition, the low yield-scaled CH 4 emissions from waterlogging demonstrate that this method effectively increases agricultural production.

As observed, high CH 4 emissions were found during the off-sowing period compared to the rice-growing period under RS water logging. Though the methanogenesis fermentation can be limited by high O 2 concentrations ( Mowjood & Kasubuchi, 1998), the root mass of the first crop could generate methanogenesis ( Jiang et al., 2019). This could be partly explained by the high CH 4 emissions during off-growing. Thus, the development of agricultural technologies to reduce CH 4 emissions during off-sowing should be performed in future research. For instance, alternative wetting and drying (AWD) or intermittent irrigation could be a suitable option for reducing CH 4 emissions because it transmits the paddy field's surface condition from reduction to oxidation by frequent contact with the air. Furthermore, the effects on the proportion of rice straw returning and seasonal carbon accumulation have not been deduced. Thus, future works should continuously examine their impacts on CH 4 emissions in the long-term run.

 Conclusions

We evaluated the effects of RS treatment measures on CH 4 emissions under waterlogging and incorporation. Our results indicated that RS decomposition under the waterlogging approach reduces CH 4 emissions compared to the incorporation approach, confirming the feasibility of RS waterlogging as a mitigation option for paddy CH 4 emissions in the VMD. Our study found that waterlogging treatment significantly contributes to CH 4 emissions during the off-sowing period. Thus, we recommend further studies by developing technologies related to water management regimes during the off-sowing period to better CH 4 emission reduction.

 Data availability Underlying data

Figshare: Methane emission in waterlogging double cropping. https://doi.org/10.6084/m9.figshare.11987628.v1 ( Oda et al., 2020).

This project contains the following underlying data:

Methane concentration and GHG_12 March 2020_17h25.xlsx (This file provides raw data that collected during experimental operation used for calculating CH 4 emissions (mgCH 4.m -2.h -1) and cumulative methane emissions (kgCH 4.ha -1.crop -1), and yield-scaled CH 4 emissions (kg CH 4 grain -1)).

Data are available under the terms of the Creative Commons Zero "No rights reserved" data waiver (CC0 1.0 Public domain dedication).

 Acknowledgments

We would like to thank Ms. Nguyen Ngoc Ngan, Mr. Ta Quang Khoi, and Mr. Nguyen Thien Thanh, students in Can Tho University, for their support of the study. We also thank Dr. Nigel Downes for proofreading the manuscript.

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Paddy and Water Environment. 2021;19:623–634. 10.1007/s10333-021-00861-8 Vo TBT Wassmann R Tirol-Padre A : Methane emission from rice cultivation in different agro-ecological zones of the Mekong river delta: seasonal patterns and emission factors for baseline water management. Soil Sci Plant Nutr. 2018;64(1):47–58. 10.1080/00380768.2017.1413926 Wang H Shen M Huic D : Straw incorporation influences soil organic carbon sequestration, greenhouse gas emission, and crop yields in a Chinese rice ( Oryza sativa L.) wheat ( Triticum aestivum L.) cropping system. Soil Tillage Res. 2019;195:104377. 10.1016/j.still.2019.104377 10.5256/f1000research.135253.r231563 Reviewer response for version 2 Islam S.M. Mofijul 1 Referee https://orcid.org/0000-0001-6236-9258 Soil Science Division, Bangladesh Rice Research Institute, Gazipur, Bangladesh

Competing interests: No competing interests were disclosed.

 9 2 2024 Copyright: © 2024 Islam SMM 2024 This is an open access peer review report distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. recommendation approve-with-reservations

In Introduction, pls write a few sentences on AWD with CH4 emissions with citation [2,1] as you mentioned it in Material and Methods section.

For a better scientific explanation, pls mention water potential data or water depth data as your expt associated with AWD.

The experimental duration was only 5 months (April to Aug 2019) which makes it very difficult to draw a conclusion.

Pls mention initial physicochemical properties of soil.

What kinds and amount of fertilizer you used for conducting this expt?

Pls mention the experimental design

Grain yield was too low. Is it meet the national demand?

For instance, alternative wetting and drying (AWD) or intermittent irrigation could be a suitable option for reducing CH4 emissions because it transmits the paddy field’s surface condition from reduction to oxidation by frequent contact with the air. Pls use ref here [1,2]

In the conclusion, you simply mentioned the results. Pls improve this section by mentioning the future direction and limitations of the experiment.

Is the work clearly and accurately presented and does it cite the current literature?

Yes

If applicable, is the statistical analysis and its interpretation appropriate?

Partly

Are all the source data underlying the results available to ensure full reproducibility?

Yes

Is the study design appropriate and is the work technically sound?

Yes

Are the conclusions drawn adequately supported by the results?

Partly

Are sufficient details of methods and analysis provided to allow replication by others?

Partly

Reviewer Expertise:

GHG emissions from rice fields, Soil fertility and plant nutrition, water management

I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard, however I have significant reservations, as outlined above.

References : Mitigating greenhouse gas emissions from irrigated rice cultivation through improved fertilizer and water management. J Environ Manage .2022;307: 10.1016/j.jenvman.2022.114520 114520 35066193 10.1016/j.jenvman.2022.114520 : Effects of water management on greenhouse gas emissions from farmers' rice fields in Bangladesh. Sci Total Environ .2020;734: 10.1016/j.scitotenv.2020.139382 139382 32460078 10.1016/j.scitotenv.2020.139382 10.5256/f1000research.135253.r231617 Reviewer response for version 2 Sriphirom Patikorn 1 Referee Department of Environmental Science, Faculty of Science, Silpakorn University, Bangkok, Bangkok, Thailand

Competing interests: No competing interests were disclosed.

 2 2 2024 Copyright: © 2024 Sriphirom P 2024 This is an open access peer review report distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. recommendation approve-with-reservations

This work in not new, but I think that it can be approved if it was appropriately designed.

I would like to receive further clarification and explanation before deciding to accept.

I have concerns and questions as follows. The author kindly answers and expresses opinions.

1. The authors should measure CH4 emissions during the off-sowing stage more than one time for RS incorporation treatment, even its duration was only 5 days. This will increase the accuracy of the cumulative emission estimates, particularly when comparing with another treatment that has longer test period. 

2. Each time of gas sampling, the authors have taken only 2 samples (3 and 23 mins after chamber closure). According to Minamikawa et al. (2015) protocol recommended CH4 sampling by collecting three samples (or more) per 20–30 min chamber deployment.

3. Why did the authors collected gas samples during 07:00-10:00 am?. There is concern that samples collected during 07:00-08:00 am will show lower emission than the mean of a day.

4. Why was the crop duration of RS incorporation (85 days) shorter than RS waterlogging practice (105 days)?.

5. “Thus, the development of agricultural technologies to reduce CH4 emissions during off-sowing should be performed in future research. For instance, alternative wetting and drying (AWD) or intermittent irrigation could be a suitable option for reducing CH4 emissions because it transmits the paddy field's surface condition from reduction to oxidation by frequent contact with the air.”

I think that the above suggestion is vague because AWD will change soil conditions that alter the mechanism of RS decomposition during off-sowing stage under RS waterlogging. Thus, the potential for CH4 mitigation during rice growing season may change and is not shown as in this current study.

6. It appeared that RS waterlogging tends to achieve higher grain yield than RS incorporation, why was this so?

7. I understand the authors that aimed to test the effects of RS practices; however, the authors need to think about farmer’s actual practices due to the authors have concluded that RS waterlogging is feasible to use as a mitigation option for paddy CH4 emissions in the VMD. It would be better if the authors increase the number of treatments to see the effect of different RS practices with fertilizer to present the effects of CH4 mitigation with yield increase or maintaining. I speculated that the obtained yield in this study is lower than the conventional practice of farmers that using chemical fertilizer. The positive results will reflect the possibility of using this CH4 mitigation technology. Therefore, the author should discuss and summarize how farmers can use this technology appropriately or what additional studies should be done to ensure appropriate use by farmers.

Is the work clearly and accurately presented and does it cite the current literature?

Partly

If applicable, is the statistical analysis and its interpretation appropriate?

Yes

Are all the source data underlying the results available to ensure full reproducibility?

Partly

Is the study design appropriate and is the work technically sound?

No

Are the conclusions drawn adequately supported by the results?

Partly

Are sufficient details of methods and analysis provided to allow replication by others?

Partly

Reviewer Expertise:

Greenhouse gas emissions and mitigation from agricultural activities.

I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard, however I have significant reservations, as outlined above.

References : Methane and nitrous oxide emissions from conventional and modified rice cultivation systems in South India. Agriculture, Ecosystems & Environment .2018;252: 10.1016/j.agee.2017.10.014 148-158 10.1016/j.agee.2017.10.014 10.5256/f1000research.135253.r143826 Reviewer response for version 2 Oikawa Yosei 1 Referee Department of International Innovative Agricultural Science, Tokyo University of Agriculture andTechnology, Tokyo, Japan

Competing interests: No competing interests were disclosed.

 17 8 2022 Copyright: © 2022 Oikawa Y 2022 This is an open access peer review report distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. recommendation approve-with-reservations

The authors have responded to all my suggestions and recommendations. I would like to confirm the following additional points to the authors. 

1. Methods of Abstract:

"Yield-scaled CH 4 emission was assessed based on total methane emission and rice yield."

I understand the authors' focus on yield-scaled emission here. But if these two treatments are applied in double cropping and triple cropping systems in one year, how different will the time-scaled methane emissions be? This is just a comment. 

2. Methods, Experimental design:

"The amount of RS applied for all treatments was the exact amount collected in the container of the first crop, correspondingly. On average, applied RS for the treatments of waterlogging and incorporation was 0.72 kg (dry weight) m -2 and 0.59 kg (dry weight) m -2, respectively." 

The different RS inputs may result in different grain yields and CH 4 emissions. Do you think the (about 20%) difference of RS amount between the treatments was negligible for data analysis and discussion? 

3. Results, Accumulative emissions: (on the Reply to Comment 9 of ver. 1)

"The total CH4 emissions from the waterlogging and incorporation treatments were 502 ± 111.4 kg CH 4 ha -1 crop -1 and 604 ± 41.9 kg CH 4 ha -1 crop -1, respectively."

On waterlogging treatment in Figure 1 (a) and (b), the sum of CH 4 emissions (217.60 and 271.93) is not equal to the total CH 4 emission, 502± 111.4 kg CH 4 ha -1 crop -1. How it was calculated? 

4. Range bars in Figure 1 to 3:

Do the range bars (or error bars) in the figures mean SD or SE?

5. The results of t-test in Figure 1 and 2:

As the authors described in the Results, the results of t-test on Figure 2(a) and (b) were not significantly different. How about the Figure 1(a) and (b)? If necessary, please add the information in the Results (or please mention the results of t-test in figures by following the editor/guidelines).

6. Water management in Figure 3:

It might be better to add "water depth", "water level", or other appropriate words in each bottom of Figure 3 (a) and (b). 

7. Discussion:

"In addition, the low yield-scaled CH 4 emissions from waterlogging demonstrate that this method effectively increases agricultural production." 

Please add the reference. Is this statement based on the grain yield of Figure 2(a)? Considering its longer cropping period (and more RS input) of the waterlogging method, it is difficult to say the underlined statement. 

8. (on the Reply to Comment 13 of ver. 1):

" As observed, high CH 4 emissions were found during the off-sowing period compared to the rice-growing period under RS water logging...

This could be partly explained by the high CH 4 emissions during off-growing."

The first sentence can be revised as "High CH 4 emissions were observed during..."

The last word should be off-sowing.  

Thank you.

Is the work clearly and accurately presented and does it cite the current literature?

Yes

If applicable, is the statistical analysis and its interpretation appropriate?

Yes

Are all the source data underlying the results available to ensure full reproducibility?

Partly

Is the study design appropriate and is the work technically sound?

Yes

Are the conclusions drawn adequately supported by the results?

Yes

Are sufficient details of methods and analysis provided to allow replication by others?

Partly

Reviewer Expertise:

My area of expertise is technical improvements and extension of sustainable agricultural and agroforestry systems.

I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard, however I have significant reservations, as outlined above.

 10.5256/f1000research.121920.r129417 Reviewer response for version 1 Tariq Azeem 1 Referee https://orcid.org/0000-0002-6132-2391 Department of Plant and Environmental Sciences, University of Copenhagen, Copenhagen, Denmark

Competing interests: No competing interests were disclosed.

 23 5 2022 Copyright: © 2022 Tariq A 2022 This is an open access peer review report distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. recommendation reject

The authors of ' Rice straw decomposition in paddy surface water potentially reduces soil methane (CH4) emission' present an effort in assessing the effects of two rice straw amendments on soil CH4 emissions during the crop growing period and during the straw decomposition.

In general the paper has a major issue with respect to the methodological approach used for sampling and comparing the two treatments of rice straw amendments. Authors did not use the uniform pattern for sampling in both treatments which could lead to overestimate/underestimate of emission in treatments. For example, in comparison for off sowing periods, authors just took one sample in soil incorporated treatment, while three in flooded treatment. Further, sampling was continued for 125 days in waterlogging, but 90 days in soil incorporated treatment. The comparison between treatments will be biased if they don’t follow the similar pattern of sampling and analysis.

Last two sentences of method section in Abstract are not clear. E.g.

“However, RS waterlogging is caused by high emissions during the off-sowing stage. The difference between yield-scaled CH4 emissions was insignificant.”

It is not clear what the authors want to explain here and what they are comparing?

Introduction section lacks clear objectives and state-of-the-art of the study. Authors also explain the results already in the introduction.

Space between words is missing at the number of places, e.g. “decomposingRS” in Introduction, units in results section etc.

Is the work clearly and accurately presented and does it cite the current literature?

Partly

If applicable, is the statistical analysis and its interpretation appropriate?

Partly

Are all the source data underlying the results available to ensure full reproducibility?

No

Is the study design appropriate and is the work technically sound?

No

Are the conclusions drawn adequately supported by the results?

No

Are sufficient details of methods and analysis provided to allow replication by others?

Partly

Reviewer Expertise:

sustainable crop production, Greenhouse gas emissions, climate change mitigation, nutrient cycling

I confirm that I have read this submission and believe that I have an appropriate level of expertise to state that I do not consider it to be of an acceptable scientific standard, for reasons outlined above.

Huynh Van Thao College of Environment and Natural Resources, Cantho Univeristy, Vietnam

Competing interests: No competing interests were disclosed.

 17 6 2022

Thank you very much for giving us constructive comments that helped us improve this paper immensely. All your comments have been carefully read and revised to elucidate underlying aspects. Our responses to your comments are as follows:

Comment 1: In general, the paper has a major issue with respect to the methodological approach used for sampling and comparing the two treatments of rice straw amendments. Authors did not use the uniform pattern for sampling in both treatments which could lead to overestimate/underestimate of emission in treatments. For example, in comparison for off sowing periods, authors just took one sample in soil incorporated treatment, while three in flooded treatment. Further, sampling was continued for 125 days in waterlogging, but 90 days in soil incorporated treatment. The comparison between treatments will be biased if they don’t follow the similar pattern of sampling and analysis.

Reply : We thank you for your comments . Actually, rice straw waterlogging is a method that could be used for a double rice cropping season in the Vietnamese Mekong Delta (VMD), while Rice straw incorporation treatment is a typical rice cultivation practice in the triple cropping season. Triple rice cropping season requires a short-time treatment of rice straw (5 days after harvesting the previous crop), while the double-cropping season has more time to treat rice straw. Thus, our study recommended 20 days of rice straw treatment for double rice cropping season. Moreover, the suggestion is based on our previous study (Thao et al. 2019, shown in the reference) conducted at a rice paddy field. It showed that 20 days were suitable for starting the second crop. Moreover, our study was conducted to test the hypothesis of whether rice straw treatment by waterlogging could be a feasible method that could be applied for the double cropping season or not. Because the primary difference of the 2 methods was the timing of the rice straw to be treated. Thus, we could accept the temporal disparity for evaluating the CH 4 emission because the comparison is carried for 2 patterns that are not similar to common ones. The difference in gas samples collected during the off-showing period depends on the time that RS was treated. We discussed deciding the frequency of samples to be taken during off-growing based on our real experiment, although there was no specific instruction for sampling during the off-growing period. We acknowledged that more frequent gas sampling could increase accuracy and reduce biases when examining methane emissions. Based on your comments and another reviewer, we adopted them and significantly revised/rearranged them logically so that it could offer a clearer version to the readers. Please find out in the Methods section.

Comment 2: Last two sentences of method section in Abstract are not clear. E.g.

“However, RS waterlogging is caused by high emissions during the off-sowing stage. The difference between yield-scaled CH 4 emissions was insignificant.” It is not clear what the authors want to explain here and what they are comparing?

Reply : We thank you for the comment. It has just been revised to make it clearer to readers. The revision is as follows “RS waterlogging is a feasible option to alternate conventional RS incorporation toward lower CH 4 emissions from rice production. Ameliorating CH 4 emission mitigation by RS waterlogging during off-sowing is recommended for future works.”

Comment 3: Introduction section lacks clear objectives and state-of-the-art of the study. Authors also explain the results already in the introduction.

Reply: Thank you so much. We revised the introduction to offer clear objectives and ameliorate the study's significance.

Comment 4: Space between words is missing at the number of places, e.g. “decomposingRS” in Introduction, units in results section etc.

Reply: We corrected it through the paper.

Thank you so much for all your comments. Looking forward to hearing from you.

Best regards,

Authors.

 10.5256/f1000research.121920.r135051 Reviewer response for version 1 Oikawa Yosei 1 Referee Department of International Innovative Agricultural Science, Tokyo University of Agriculture andTechnology, Tokyo, Japan

Competing interests: No competing interests were disclosed.

 16 5 2022 Copyright: © 2022 Oikawa Y 2022 This is an open access peer review report distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The author(s) is/are employees of the US Government and therefore domestic copyright protection in USA does not apply to this work. The work may be protected under the copyright laws of other jurisdictions when used in those jurisdictions. recommendation approve-with-reservations

This study investigated the effects of rice straw waterlogging and rice straw incorporation into the soil on methane emission through a microcosm experiment. The results showed significant differences between those treatments and seem to contain some suggestive points to reduce methane emissions in paddy fields. However, there are unclear points in the study. To improve the current version 1, I would like to suggest or recommend the following questions and comments:

 

Please define "off-sowing periods" in Methods.

"Thao et al. (2019) demonstrated a field experiment on RS decomposing in field surface water 20 days before sowing as a suitable possibility for developing a double-cropping pattern in the Mekong Delta." It is difficult for me to understand this sentence, especially "suitable possibility". 

Some descriptions in sub-chapters "Study setting and materials", "Rice cultivation", "Experimental design", and "Measurements" can be moved to more appropriate places. 

I recommend the authors to briefly explain the reason of no fertilizer application. Is "no fertilizer cultivation" commonly practiced in the Mekong Delta? If any fertilizer was applied, what would happen in the experiment except yield increase? Can local farmers practice both "no fertilizer cultivation" and fertilizer application for their profit?

I recommend the authors to elaborate the microcosms used in the experiment such as size and shape. 

I recommend the authors to elaborate "RS incorporation into soil" such as the soil depth and the amount of rice straw (that must be the same amount as that of harvested from the first cropping and that of waterlogged treatment). Is the RS  incorporation into soil a conventional method in the Mekong delta? An additional sentence would be appreciated to explain the reason why this treatment can be the control to RS waterlogged treatment. 

"Rice-growing period" and "off-sowing period/stage" need to be more clearly defined. Are off-sowing periods 20 days for waterlogged treatment, 5 days for incorporation treatment?

Significant digits of some data need to be confirmed. For example, on "0.21 ± 0.02 kg CH 4.kg grain -1 and 0.3 ± 0.08 kg CH 4. kg grain -1", 0.3 might be 0.30. 

Data Fig.1 and Fig. 2 were summed to describe the total CH4 emission. It would be better to kindly add the data in figure to the descriptions on Fig. 1a and Fig. 1b before showing the total CH4 emission, "502 ± 111.4 kgCH4.ha-1.crop-1 and 604 ± 41.9 kgCH4.ha-1.crop-1". 

It is recommended that "rice-growing period" and "off-sowing period" are shown in Figure 3a and 3b. 

I think the data on rice yields of both treatments are necessary for further discussion, as well as rice straw inputs. 

"The efficiency of CH4 emission from rice straw waterlogging on the field surface promotes aerobic decomposition, resulting in reducing CH 4 emission." Does it mean that other carbon sources such as CO 2 and organic carbon increased or released? 

"when waterlogging is used during the off-sowing period, CH4 emission is more concentrated than incorporation because the rice straw decomposition period is longer than that of rice straw incorporation treatment." The reason is not clear to me. Does it mean that rice straw in water is slowly decomposed in an anaerobic condition as compared to that incorporated with soil? I would appreciate it if you could kindly explain the reason of high methane emission during the off-sowing period (and if any possible countermeasures to avoid it). 

Is the work clearly and accurately presented and does it cite the current literature?

Yes

If applicable, is the statistical analysis and its interpretation appropriate?

Yes

Are all the source data underlying the results available to ensure full reproducibility?

Partly

Is the study design appropriate and is the work technically sound?

Yes

Are the conclusions drawn adequately supported by the results?

Yes

Are sufficient details of methods and analysis provided to allow replication by others?

Partly

Reviewer Expertise:

My area of expertise is technical improvements and extension of sustainable agricultural and agroforestry systems.

I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard, however I have significant reservations, as outlined above.

Huynh Van Thao College of Environment and Natural Resources, Cantho Univeristy, Vietnam

Competing interests: No competing interests were disclosed.

 17 6 2022

Thank you very much for giving us constructive comments that helped us improve this paper immensely. All your comments have been carefully read and revised to elucidate underlying aspects. Our responses to your comments are as follows:

Comment 1: Please define "off-sowing periods" in Methods.

Reply 1 : We added the term in the methods. Please find out in the experimental design.

Comment 2: "Thao et al. (2019) demonstrated a field experiment on RS decomposing in field surface water 20 days before sowing as a suitable possibility for developing a double-cropping pattern in the Mekong Delta." It is difficult for me to understand this sentence, especially "suitable possibility".

Reply : We revised this sentence to make it more apparent to readers. The revision is as follows “In a field experiment, Thao et al. (2019) reported that a 20-day period of RS decomposition in surface water (off-sowing) was suitable for growing rice in the VMD's double-cropping pattern”. Please find out in the introduction section.

Comment 3: Some descriptions in sub-chapters "Study setting and materials", "Rice cultivation", "Experimental design", and "Measurements" can be moved to more appropriate places.

Reply : We appropriately revised the method, these subitems in the methods have been rearranged more appropriate. Please find out in the methods section. The title of “Study setting and materials” has been changed to “Experimental site and materials” and the “rice cultivation” was integrated into the “Experimental design”.

Comment 4: I recommend the authors to briefly explain the reason of no fertilizer application. Is "no fertilizer cultivation" commonly practiced in the Mekong Delta? If any fertilizer was applied, what would happen in the experiment except yield increase? Can local farmers practice both "no fertilizer cultivation" and fertilizer application for their profit?

Reply : Our study set the study’s aim of examining the effect of the rice straw treating approach by waterlogging and incorporation to methane emission patterns. Fertilizers are known as a primary factor that could significantly change the CH 4 emission patterns. Therefore, we did not apply fertilizers for the treatments to eliminate unexpected impacts on the findings. We added a brief explanation in the method section for the updated version.

As you inquired, it is acknowledged that fertilization has a common practice of most farmers in the Vietnamese Mekong Delta with expect to increase rice yield; no fertilization is also considered for organic rice practices in VMD. However, comparing the practice of no fertilizer cultivation" and fertilizer application in the same pattern has not been disclosed.

Comment 5: I recommend the authors to elaborate the microcosms used in the experiment such as size and shape.

Reply : It was added in the material section of the Methods.

Comment 6: I recommend the authors to elaborate "RS incorporation into soil" such as the soil depth and the amount of rice straw (that must be the same amount as that of harvested from the first cropping and that of waterlogged treatment). Is the RS incorporation into soil a conventional method in the Mekong delta? An additional sentence would be appreciated to explain the reason why this treatment can be the control to RS waterlogged treatment.

Reply : Thank you so much for your recommendation. Your recommendations have been adopted. We added 2 additional sentences to make it more straightforward for readers. It is as follows “The soil depth was mixed approximately 20 cm in depth. Then, it … (off-sowing period). RS incorporation into the soil treated for 5 days was a typical treatment pattern for a triple-cropping rice production system in the VMD. The amount of RS applied for treatments was the same amount collected in the container, correspondingly.” We added the measured data to the methods as well as discussion section.

Comment 7: "Rice-growing period" and "off-sowing period/stage" need to be more clearly defined. Are off-sowing periods 20 days for waterlogged treatment, 5 days for incorporation treatment?

Reply : We clarified the term of off-sowing period in the method section. It is as follows “The term of off-sowing used indicates a period that RS treated before sowing, which was 20 days for the RS waterlogging treatment and 5 days for the RS incorporation treatment.”

Comment 8: Significant digits of some data need to be confirmed. For example, on "0.21 ± 0.02 kg CH 4. kg grain -1 and 0.3 ± 0.08 kg CH 4. kg grain -1 ", 0.3 might be 0.30.

Reply : We confirm the number of digits after the decimal point as commented.

Comment 9: Data Fig.1 and Fig. 2 were summed to describe the total CH 4 emission. It would be better to kindly add the data in figure to the descriptions on Fig. 1a and Fig. 1b before showing the total CH 4 emission, "502 ± 111.4 kg CH 4.ha -1.crop -1 and 604 ± 41.9 kg CH 4.ha -1.crop -1".

Reply : Your suggestion has been adopted. Please find put these Figures.

Comment 10: It is recommended that "rice-growing period" and "off-sowing period" are shown in Figure 3a and 3b.  

Reply : Your suggestion has been adopted. Revision has been shown in the Figure.

Comment 11: I think the data on rice yields of both treatments are necessary for further Discussion, as well as rice straw inputs.

Reply : AS mentioned, the brief report aimed to test the hypothesis of whether the rice straw waterlogging method could reduce methane compared with conventional RS treatment by incorporation or not. Previously, it was suggested that discussion should emphasize the methane emissions rather than rice yield and rice straw application, although rice yield and amount of rice straw added were also recorded in this study. However, your recommendations have been adopted to elucidate underlying aspects to readers. We added Figure 2a to show the yield between 2 treatments. Also, the amount of rice straw also added to the discussion section.

Comment 12: "The efficiency of CH 4 emission from rice straw waterlogging on the field surface promotes aerobic decomposition, resulting in reducing CH 4 emission." Does it mean that other carbon sources such as CO 2 and organic carbon increased or released?

Reply : It is well known that decomposing of organic matters in aerobic conditions produces much more CO 2 due to microbial heterotrophic respiration. To clarify, we revised that sentence as follows: “The efficiency of CH 4 emission mitigation from RS waterlogging on the field surface was more likely attributed to the RS decomposition in an aerobic environment because CH 4 formation is more favourable in anaerobic conditions”.

Comment 13: when waterlogging is used during the off-sowing period, CH 4 emission is more concentrated than incorporation because the rice straw decomposition period is longer than that of rice straw incorporation treatment." The reason is not clear to me. Does it mean that rice straw in water is slowly decomposed in an anaerobic condition as compared to that incorporated with soil? I would appreciate it if you could kindly explain the reason of high methane emission during the off-sowing period (and if any possible countermeasures to avoid it). 

Reply : thank you so much for your question as well as your recommendation. We revised and added additional explanations to make it clearer to readers. The revision is as follows: “As observed, high CH 4 emissions were found during the off-sowing period compared to the rice-growing period under RS water logging. Though the methanogenesis fermentation can be limited by high O 2 concentrations ( Mowjood & Kasubuchi, 1998), the root mass of the first crop could generate methanogenesis ( Jiang et al., 2019). This could be partly explained by the high CH 4 emissions during off-growing”.

Thank you so much for all your comments. Looking forward to hearing from you.

Best regards,

Authors.