Rice straw decomposition in paddy surface water potentially reduces soil methane (CH<sub>4</sub>) emission

Huynh Van Thao; Masato Oda; Nguyen Huu Chiem

doi:10.12688/f1000research.110326.1

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Rice straw decomposition in paddy surface water potentially reduces soil methane (CH₄) emission

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

Huynh Van Thao¹, Masato Oda², Nguyen Huu Chiem¹

PUBLISHED 10 Mar 2022

Author details Author details

¹ College of Environment and Natural Resources, Can Tho University, Can Tho city, 900000, Vietnam
² Japan International Research Center for Agricultural Sciences, Tsukuba city, 305-8686, Japan

Huynh Van Thao
Roles: Conceptualization, Data Curation, Formal Analysis, Methodology, Writing – Original Draft Preparation

Masato Oda
Roles: Conceptualization, Methodology, Supervision, Validation, Writing – Review & Editing

Nguyen Huu Chiem
Roles: Conceptualization, Methodology, Supervision, Writing – Review & Editing

OPEN PEER REVIEW

REVIEWER STATUS

This article is included in the Agriculture, Food and Nutrition gateway.

Abstract

Background: Rice cultivation is a significant methane (CH₄) emission source. Rice straw (RS) incorporation into the soil is a key factor that produces higher CH₄ emission. The RS waterlogging approach on the soil surface possibly reduces CH₄ emission due to not being buried into the soil. However, evaluation of CH₄ emission by this approach has not been determined. The objective of this study was to examine CH₄ emission 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 CH₄ emissions and CH₄accumulation for the rice-growing and off-sowing periods. Yield-scaled CH₄ emission was assessed based on total methane emission and rice yield.
Results: The results demonstrated that RS waterlogging reduced CH₄ emission by 16.9% compared to RS incorporation into the soil. During the rice-growing period, total CH₄emission from RS waterlogging accounted for 36% of the incorporation treatment. However, RS waterlogging is caused by high emissions during the off-sowing stage. The difference between yield-scaled CH₄ emissions was insignificant.
Conclusions: This study demonstrated that the treatment of RS by waterlogging is an appropriate alternative to conventional RS practices known as incorporation, which increases greenhouse CH₄ emission. However, high CH₄ emission during the off-sowing period, and RS accumulation in the field are key drivers that possibly contribute to greenhouse gas emissions. Therefore, further evaluation is needed to determine the long-term effects of this approach.

Keywords

greenhouse gas emission, rice straw incorporation, methane emission, organic rice, water management, the Vietnamese Mekong Delta

Corresponding author: Nguyen Huu Chiem

Competing interests: No competing interests were disclosed.

Grant information: The author(s) declared that no grants were involved in supporting this work.

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

How to cite: Van Thao H, Oda M and Chiem NH. Rice straw decomposition in paddy surface water potentially reduces soil methane (CH₄) emission [version 1; peer review: 1 approved with reservations, 1 not approved]. F1000Research 2022, 11:298 (https://doi.org/10.12688/f1000research.110326.1) First published: 10 Mar 2022, 11:298 (https://doi.org/10.12688/f1000research.110326.1) Latest published: 11 Jul 2022, 11:298 (https://doi.org/10.12688/f1000research.110326.2)

Introduction

The agricultural sector contributes approximately 10–12% of global anthropogenic emissions. Of these emissions, 47% of methane (CH₄) emission has been attributed to agriculture production (Smith et al., 2007). Rice fields have been considered an important source of atmospheric CH₄, accounting for 15–20% of the global total anthropogenic CH₄ emission (Sass & Fisher, 1997). Rice straw (RS) plays a vital role in contributing to CH₄ emission from paddy fields. Several field experiments have shown that RS incorporation significantly increases CH₄ emissions in rice fields (Hoa et al., 2019; Jiang et al., 2019; Liu et al., 2015; Wang et al., 2019). In the Mekong Delta of Vietnam, the intensification of rice cultivation is a great contributor to CH₄ emission (Oda & Chiem, 2019). Several studies have explored decomposing RS in paddy surface water as an effective pathway to reduce CH₄ emission (Boateng et al., 2017; Oda & Chiem, 2019; Tariq et al., 2017). For example, 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. However, the research did not determine how much this process reduced CH₄ emissions. In addition, Oda & Chiem (2019) suggested that the strategy of decomposing RS on surface water effectively reduces CH₄ emission from the paddies. To the best of our knowledge, CH₄ emission from the RS decomposition process in water has not been thoroughly studied. Therefore, we conducted a microcosms experiment to clarify whether RS waterlogging reduces CH₄ emission, and the results found that this approach did decrease the CH₄ emission from the paddy field.

Methods

Study setting 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. The inner humidity in the experimental house is relatively similar compared with outdoors. We used plastic containers (38 cm × 58 cm × 30 cm high) filled to 20 cm with paddy field soil (10°18′N, 105°54′E). The soil was classified as Thionic Glycesol (International Union of Soil Sciences (IUSS) working group World Reference Base (WRB), 2015) (Dong et al., 2012).

Rice cultivation

Effects of RS waterlogging and RS incorporation into the soil on methane emission were evaluated following a conventional rice cultivation experiment (first crop). The first experiment set essential conditions (rice straw, soil) for implementing the second experiment. In the first crop, a short-duration variety of rice was used (IR50404 cultivar, 85–90 days) which is a typical rice variety of Vietnam, provided by Cuu Long Delta Rice Research Center (CLRRI). Pre-germinated seeds were sown at an equivalent rate of 250 kg ha^-1 on wet-leveled soil. 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 (Hoa et al., 2018). In the second crop, we used RS and soil from containers in the first crop to examine the effects of rice straw incorporation and waterlogging on methane emission. Fresh RS (above-surface biomass) in the first crop was collected and cut into 5 cm in length. Then it was immediately scattered onto the soil surface as well as incorporated into the soil, correspondingly. The IR50404 variety was also sown at a rate of equivalent to 250 kg ha^-1. Water irrigation was managed as a continuously flooding management method during the rice-growing period. Fertilizers were not applied for either experiment.

Experimental design

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. A total of six microcosms were laid out closely in an array of two columns and three rows. Both treatments were performed one day after harvesting the first crop.

In the waterlogging treatment, RS was scattered on the soil surface and irrigated to 10 cm in-depth for the waterlogging treatment. Then, the RS was gently pressed into water. It was left for a 20-day stage without disturbing (off-sowing period). This timing was followed by a field demonstration recommended by Thao et al. (2019) that RS was well-fermented in water within 20 days, and the rice field has suitable conditions for broadcasting rice seeds. In the RS incorporation treatment, RS was incorporated into the soil by a shovel and immediately irrigated to 2 cm in-depth for a 5-day period (off-sowing period), a typical treatment pattern for a triple-cropping rice production system in the Vietnamese Mekong Delta.

On the sowing day, we drained the field and leveled it by hand. The soil was not reincorporated. 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.

Measurements

The closed chamber method was used to measure CH₄ 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 50mL at 3, and 23 min after the chamber placement, and each gas sample was immediately injected to 15 mL in vacuumed vials. During the off-sowing period (rice straw treated before harvesting), 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. 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 CH₄ concentration was analyzed by gas chromatography (Shimadzu GC2014, Japan) equipped with a flame ionization detector, using 60/80 Carboxen® 1000 column at temperature 180 °C. Nitrogen (99.99%) as a carrier gas at a flow rate of 30 ml min⁻¹.

Tap water was directly irrigated for rice containers. Water levels were checked by a 50-cm ruler (1-mm scale). Grain yield was detected by harvesting all rice in each pot and removing all unfilled grains using tap water. Grains were 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 CH₄ 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. Yield-scale CH₄ emission was calculated by dividing total methane emission by grain yield. Detailed guidance can be found at Minamikawa et al. (2015). All measurements were carried out with three repetitions. Data processes were performed using Microsoft Excel 2019.

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 CH₄ emission in rice-growing and off-sowing periods as well as yield-scaled CH₄ emission. The statistical significance was done with a confident level of 95%.

Results

Accumulative emission

We assessed the effects of rice straw management via waterlogging and incorporation on CH₄ emission using a container experiment. The results showed that the CH₄ 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 (Figure 1b). The total CH₄ emissions from the waterlogging and incorporation treatments were 502 ± 111.4 kgCH₄.ha^-1.crop^-1 and 604 ± 41.9 kgCH₄.ha^-1.crop^-1, respectively. In general, the magnitude of seasonal CH₄ emission that was observed in our study was lower than what was found in previous studies on triple-cropping in the central Mekong Delta of Vietnam, which ranged between 710 and 1,789 kgCH₄.ha^-1.crop^-1 (Oda & Chiem, 2019; Vo et al., 2018). The CH₄ 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. For the yield-scaled CH₄ emission from straw, the waterlogging and incorporation models were 0.21 ± 0.02 kg CH₄.kg grain^-1 and 0.3 ± 0.08 kg CH₄. kg grain^-1, respectively. The difference between yield-scaled CH₄ emissions was insignificant (P>0.05) (Figure 2).

Figure 1.

CH₄ emission accumulation of rice strawy (RS) waterlogging and RS incorporation in the periods of rice-growing (a), and off-sowing (b).

Figure 2. Yield-scaled CH₄ emission of rice straw (RS) waterlogging and RS incorporation.

Emission pattern

CH₄ emission 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 CH₄ emission of the incorporation approach was always higher when compared with the waterlogging method. The first peak was in line with a previous study (Oda & Chiem, 2019).

Figure 3. CH₄ emission rate of rice straw (RS) waterlogging and RS incorporation.

Discussion

Conventional rice cultivation based on rice straw incorporation of paddy fields is a substantial source of CH₄ emissions. Modification of rice straw practices is undoubtedly necessary to reduce CH₄ flux when the rice straw incorporated into the soil. In other words, rice straw incorporation will be the most detectable substrate source to contribute to higher CH₄ emission from rice paddy fields. Although rice straw amendment enriches soil organic carbon and improves soil fertility (Bjoern et al., 2014; Liu et al., 2014), it increases the availability of organic carbon and simultaneously intensifies strict anaerobic conditions to stimulate CH₄ formation on the rice paddy field (Sass et al., 1991). Waterlogging rice straw management strategy instead of incorporation demonstrates less methane emission 12%, even though rice straw applied during the fallow period decreased CH₄ emission by 11% compared to the same amount rice straw applied during rice filed preparation (Lu et al., 2000). In this study, we reached similar results in mitigation of emission from the rice straw practice with regards to non-incorporation reduced 16.9% for the whole period. The efficiency of CH₄ emission from rice straw waterlogging on the field surface promotes aerobic decomposition, resulting in reducing CH₄ emission.

This study showed the effects of rice straw management on CH₄ emission in the rice-growing period. Total CH₄ emission of rice straw after waterlogging was lower than that of the incorporation approach. These findings suggest that when rice straw was decomposed in water generated less CH₄ emission than when it was buried in the soil. This can be explained by decomposition via soil-flooding management, which blocks oxygen penetration into the soil and creates a stable anaerobic condition, allowing bacteria capable of producing CH₄ to thrive (Conrad, 2007). In the decomposition of rice straw in water, which is generally affected by dissolved oxygen, methanogenesis fermentation can be limited by high O₂ concentrations (Jiang et al., 2019). In addition, the low yield-scaled CH4 emission from waterlogging demonstrates that this method effectively increases agricultural production and improves environmental protection.

As observed in this study, 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. Specifically, the off-sowing period was conducted for 20 days, while the rice straw incorporation treatment was performed for five days only. Subsequently, the total CH₄ emission during the RS waterlogging is more significant than rice straw incorporation. Rice straw CH₄ emission sources generate from rice straw decomposing. During off-sowing, rice straw waterlogging decomposes faster than incorporation and produces much more readily available carbon. The decomposing process consumes dissolved oxygen in the water, creating an anaerobic condition, contributing to the CH₄ generation.

The development of agricultural technologies to reduce CH₄ emission 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 emission during the waterlogging period because it transmits the surface condition of the paddy field from reduction condition to oxidation condition 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 effects on CH₄ emission in the long-term run.

Conclusions

We evaluated effects of RS treatment measures on CH₄ emissions under waterlogging and incorporation. Our results indicated that RS decomposition under the waterlogging approach reduces CH₄ emission compared to the incorporation approach, confirming the feasibility of rice straw waterlogging as a mitigation option for paddy CH₄ emission in the Vietnamese Mekong Delta. However, waterlogging significantly contribute to CH₄ emission during the off-sowing period. Thus, we recommend that reducing CH₄ emission using RS decomposition during off-showing should be examined for further studies.

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 CH₄ emission (mgCH₄.m^-2.h^-1) and cumulative methane emission (kgCH₄.ha^-1.crop^-1), and yield-scaled CH₄ emission (kg CH₄ 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.

Faculty Opinions recommended

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Version 2

VERSION 2 PUBLISHED 10 Mar 2022

Author details Author details

¹ College of Environment and Natural Resources, Can Tho University, Can Tho city, 900000, Vietnam
² Japan International Research Center for Agricultural Sciences, Tsukuba city, 305-8686, Japan

Huynh Van Thao
Roles: Conceptualization, Data Curation, Formal Analysis, Methodology, Writing – Original Draft Preparation

Masato Oda
Roles: Conceptualization, Methodology, Supervision, Validation, Writing – Review & Editing

Nguyen Huu Chiem
Roles: Conceptualization, Methodology, Supervision, Writing – Review & Editing

Competing interests

No competing interests were disclosed.

Grant information

The author(s) declared that no grants were involved in supporting this work.

Article Versions (2)

version 2

Revised

Published: 11 Jul 2022, 11:298

https://doi.org/10.12688/f1000research.110326.2

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Published: 10 Mar 2022, 11:298

https://doi.org/10.12688/f1000research.110326.1

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

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Van Thao H, Oda M and Chiem NH. Rice straw decomposition in paddy surface water potentially reduces soil methane (CH₄) emission [version 1; peer review: 1 approved with reservations, 1 not approved]. F1000Research 2022, 11:298 (https://doi.org/10.12688/f1000research.110326.1)

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PUBLISHED 10 Mar 2022

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Reviewer Report 23 May 2022

Azeem Tariq, Department of Plant and Environmental Sciences, University of Copenhagen, Copenhagen, Denmark

Not Approved

https://doi.org/10.5256/f1000research.121920.r129417

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

No
Are sufficient details of methods and analysis provided to allow replication by others?

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
Are the conclusions drawn adequately supported by the results?

No

Competing Interests: No competing interests were disclosed.

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.

CITE

Report a concern

Author Response 17 Jun 2022

Thao Huynh Van, College of Environment and Natural Resources, Can Tho University, Can Tho city, 900000, Vietnam

17 Jun 2022

Author Response

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 ... Continue reading 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 CH₄ 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 CH₄ 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 CH₄ 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.
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 CH₄ 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 CH₄ 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 CH₄ 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.
Competing Interests: No competing interests were disclosed. Close
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COMMENTS ON THIS REPORT

Author Response 17 Jun 2022

Thao Huynh Van, College of Environment and Natural Resources, Can Tho University, Can Tho city, 900000, Vietnam

17 Jun 2022

Author Response

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 ... Continue reading 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 CH₄ 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 CH₄ 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 CH₄ 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.
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 CH₄ 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 CH₄ 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 CH₄ 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.
Competing Interests: No competing interests were disclosed. Close
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Reviewer Report 16 May 2022

Yosei Oikawa, Department of International Innovative Agricultural Science, Tokyo University of Agriculture andTechnology, Tokyo, Japan

Approved with Reservations

https://doi.org/10.5256/f1000research.121920.r135051

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 CH₄.kg grain^-1 and 0.3 ± 0.08 kg CH₄. 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 CH₄ emission." Does it mean that other carbon sources such as CO₂ 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
Is the study design appropriate and is the work technically sound?

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

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
Are the conclusions drawn adequately supported by the results?

Yes

Competing Interests: No competing interests were disclosed.

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.

CITE

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Author Response 17 Jun 2022

Thao Huynh Van, College of Environment and Natural Resources, Can Tho University, Can Tho city, 900000, Vietnam

17 Jun 2022

Author Response

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 ... Continue reading 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 CH₄ 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 CH₄. kg grain^-1 and 0.3 ± 0.08 kg CH₄. 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 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 kg CH₄.ha^-1.crop^-1 and 604 ± 41.9 kg CH₄.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 CH₄ emission from rice straw waterlogging on the field surface promotes aerobic decomposition, resulting in reducing CH₄ emission." Does it mean that other carbon sources such as CO₂ and organic carbon increased or released?

Reply: It is well known that decomposing of organic matters in aerobic conditions produces much more CO₂ due to microbial heterotrophic respiration. To clarify, we revised that sentence as follows: “The efficiency of CH₄ emission mitigation from RS waterlogging on the field surface was more likely attributed to the RS decomposition in an aerobic environment because CH₄ formation is more favourable in anaerobic conditions”.

Comment 13: 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).

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 CH₄ 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 CH₄ emissions during off-growing”.

Thank you so much for all your comments. Looking forward to hearing from you.
Best regards,
Authors.
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 CH₄ 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 CH₄. kg grain^-1 and 0.3 ± 0.08 kg CH₄. 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 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 kg CH₄.ha^-1.crop^-1 and 604 ± 41.9 kg CH₄.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 CH₄ emission from rice straw waterlogging on the field surface promotes aerobic decomposition, resulting in reducing CH₄ emission." Does it mean that other carbon sources such as CO₂ and organic carbon increased or released?

Reply: It is well known that decomposing of organic matters in aerobic conditions produces much more CO₂ due to microbial heterotrophic respiration. To clarify, we revised that sentence as follows: “The efficiency of CH₄ emission mitigation from RS waterlogging on the field surface was more likely attributed to the RS decomposition in an aerobic environment because CH₄ formation is more favourable in anaerobic conditions”.

Comment 13: 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).

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 CH₄ 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 CH₄ emissions during off-growing”.

Thank you so much for all your comments. Looking forward to hearing from you.
Best regards,
Authors.
Competing Interests: No competing interests were disclosed. Close
Report a concern
Respond or Comment

COMMENTS ON THIS REPORT

Author Response 17 Jun 2022

Thao Huynh Van, College of Environment and Natural Resources, Can Tho University, Can Tho city, 900000, Vietnam

17 Jun 2022

Author Response

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 ... Continue reading 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 CH₄ 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 CH₄. kg grain^-1 and 0.3 ± 0.08 kg CH₄. 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 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 kg CH₄.ha^-1.crop^-1 and 604 ± 41.9 kg CH₄.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 CH₄ emission from rice straw waterlogging on the field surface promotes aerobic decomposition, resulting in reducing CH₄ emission." Does it mean that other carbon sources such as CO₂ and organic carbon increased or released?

Reply: It is well known that decomposing of organic matters in aerobic conditions produces much more CO₂ due to microbial heterotrophic respiration. To clarify, we revised that sentence as follows: “The efficiency of CH₄ emission mitigation from RS waterlogging on the field surface was more likely attributed to the RS decomposition in an aerobic environment because CH₄ formation is more favourable in anaerobic conditions”.

Comment 13: 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).

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 CH₄ 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 CH₄ emissions during off-growing”.

Thank you so much for all your comments. Looking forward to hearing from you.
Best regards,
Authors.
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 CH₄ 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 CH₄. kg grain^-1 and 0.3 ± 0.08 kg CH₄. 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 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 kg CH₄.ha^-1.crop^-1 and 604 ± 41.9 kg CH₄.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 CH₄ emission from rice straw waterlogging on the field surface promotes aerobic decomposition, resulting in reducing CH₄ emission." Does it mean that other carbon sources such as CO₂ and organic carbon increased or released?

Reply: It is well known that decomposing of organic matters in aerobic conditions produces much more CO₂ due to microbial heterotrophic respiration. To clarify, we revised that sentence as follows: “The efficiency of CH₄ emission mitigation from RS waterlogging on the field surface was more likely attributed to the RS decomposition in an aerobic environment because CH₄ formation is more favourable in anaerobic conditions”.

Comment 13: 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).

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 CH₄ 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 CH₄ emissions during off-growing”.

Thank you so much for all your comments. Looking forward to hearing from you.
Best regards,
Authors.
Competing Interests: No competing interests were disclosed. Close
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Version 2

VERSION 2 PUBLISHED 10 Mar 2022

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Yosei Oikawa, Tokyo University of Agriculture andTechnology, Tokyo, Japan
Azeem Tariq, University of Copenhagen, Copenhagen, Denmark
Patikorn Sriphirom, Silpakorn University, Bangkok, Thailand
S.M. Mofijul Islam, Bangladesh Rice Research Institute, Gazipur, Bangladesh

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4 Views

09 Feb 2024 | for Version 2

S.M. Mofijul Islam, Soil Science Division, Bangladesh Rice Research Institute, Gazipur, Bangladesh

4 Views Cite this report Responses(0)

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

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

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?

Yes
Are the conclusions drawn adequately supported by the results?

Partly

References

1. Islam SMM, Gaihre YK, Islam MR, Ahmed MN, et al.: Mitigating greenhouse gas emissions from irrigated rice cultivation through improved fertilizer and water management.J Environ Manage. 2022; 307: 114520 PubMed Abstract | Publisher Full Text
2. Islam SMM, Gaihre YK, Islam MR, Akter M, et al.: Effects of water management on greenhouse gas emissions from farmers' rice fields in Bangladesh.Sci Total Environ. 2020; 734: 139382 PubMed Abstract | Publisher Full Text

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No competing interests were disclosed.

Reviewer Expertise

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

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4 Views

02 Feb 2024 | for Version 2

Patikorn Sriphirom, Department of Environmental Science, Faculty of Science, Silpakorn University, Bangkok, Bangkok, Thailand

4 Views Cite this report Responses(0)

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

No
Are sufficient details of methods and analysis provided to allow replication by others?

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
Are the conclusions drawn adequately supported by the results?

Partly

References

1. Oo A, Sudo S, Inubushi K, Mano M, et al.: Methane and nitrous oxide emissions from conventional and modified rice cultivation systems in South India. Agriculture, Ecosystems & Environment. 2018; 252: 148-158 Publisher Full Text

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No competing interests were disclosed.

Reviewer Expertise

Greenhouse gas emissions and mitigation from agricultural activities.

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5 Views

17 Aug 2022 | for Version 2

Yosei Oikawa, Department of International Innovative Agricultural Science, Tokyo University of Agriculture andTechnology, Tokyo, Japan

5 Views Cite this report Responses(0)

Approved 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 CH₄ 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 CH₄ 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 CH₄ ha^-1 crop^-1 and 604 ± 41.9 kg CH₄ ha^-1 crop^-1, respectively."

On waterlogging treatment in Figure 1 (a) and (b), the sum of CH₄ emissions (217.60 and 271.93) is not equal to the total CH₄ emission, 502± 111.4 kg CH₄ 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 CH₄ 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 CH₄ 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 CH₄ emissions during off-growing."

The first sentence can be revised as "High CH₄ emissions were observed during..."
The last word should be off-sowing.

Thank you.

Competing Interests

No competing interests were disclosed.

Reviewer Expertise

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

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17 Views

23 May 2022 | for Version 1

Azeem Tariq, Department of Plant and Environmental Sciences, University of Copenhagen, Copenhagen, Denmark

17 Views Cite this report Responses(1)

Not Approved

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

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

No
Are sufficient details of methods and analysis provided to allow replication by others?

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
Are the conclusions drawn adequately supported by the results?

No

Competing Interests

No competing interests were disclosed.

Reviewer Expertise

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

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Author Response

17 Jun 2022

Thao Huynh Van, College of Environment and Natural Resources, Can Tho University, Can Tho city, 900000, Vietnam

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 CH₄ 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 CH₄ 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 CH₄ 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.

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No competing interests were disclosed.

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32 Views

16 May 2022 | for Version 1

Yosei Oikawa, Department of International Innovative Agricultural Science, Tokyo University of Agriculture andTechnology, Tokyo, Japan

32 Views Cite this report Responses(1)

Approved With Reservations

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 CH₄.kg grain^-1 and 0.3 ± 0.08 kg CH₄. 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 CH₄ emission." Does it mean that other carbon sources such as CO₂ 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
Is the study design appropriate and is the work technically sound?

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

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
Are the conclusions drawn adequately supported by the results?

Yes

Competing Interests

No competing interests were disclosed.

Reviewer Expertise

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

Respond to this report

Responses (1)

Author Response

17 Jun 2022

Thao Huynh Van, College of Environment and Natural Resources, Can Tho University, Can Tho city, 900000, Vietnam

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 CH₄ 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 CH₄. kg grain^-1 and 0.3 ± 0.08 kg CH₄. 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 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 kg CH₄.ha^-1.crop^-1 and 604 ± 41.9 kg CH₄.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 CH₄ emission from rice straw waterlogging on the field surface promotes aerobic decomposition, resulting in reducing CH₄ emission." Does it mean that other carbon sources such as CO₂ and organic carbon increased or released?

Reply: It is well known that decomposing of organic matters in aerobic conditions produces much more CO₂ due to microbial heterotrophic respiration. To clarify, we revised that sentence as follows: “The efficiency of CH₄ emission mitigation from RS waterlogging on the field surface was more likely attributed to the RS decomposition in an aerobic environment because CH₄ formation is more favourable in anaerobic conditions”.

Comment 13: 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).

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 CH₄ 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 CH₄ emissions during off-growing”.

Thank you so much for all your comments. Looking forward to hearing from you.
Best regards,
Authors.

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Competing Interests

No competing interests were disclosed.

Alongside their report, reviewers assign a status to the article:

Approved - the paper is scientifically sound in its current form and only minor, if any, improvements are suggested

Approved with reservations - A number of small changes, sometimes more significant revisions are required to address specific details and improve the papers academic merit.

Not approved - fundamental flaws in the paper seriously undermine the findings and conclusions

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Rice straw decomposition in paddy surface water potentially reduces soil methane (CH4) emission

Abstract

Keywords

Introduction

Methods

Study setting and materials

Rice cultivation

Experimental design

Measurements

Data processing

Analysis

Results

Accumulative emission

Figure 1.

Figure 2. Yield-scaled CH4 emission of rice straw (RS) waterlogging and RS incorporation.

Emission pattern

Figure 3. CH4 emission rate of rice straw (RS) waterlogging and RS incorporation.

Discussion

Conclusions

Data availability

Underlying data

Acknowledgments

References

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Rice straw decomposition in paddy surface water potentially reduces soil methane (CH₄) emission

Figure 2. Yield-scaled CH₄ emission of rice straw (RS) waterlogging and RS incorporation.

Figure 3. CH₄ emission rate of rice straw (RS) waterlogging and RS incorporation.