F1000Research F1000Research 2046-1402 F1000 Research Limited London, UK 10.12688/f1000research.110326.1 Brief Report Articles Rice straw decomposition in paddy surface water potentially reduces soil methane (CH 4) emission

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

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

No competing interests were disclosed.

 10 3 2022 2022 11 298 3 3 2022 Copyright: © 2022 Van Thao H et al. 2022 This is an open access article distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Background:Rice cultivation is a significant methane (CH 4) emission source. Rice straw (RS) incorporation into the soil is a key factor that produces higher CH 4 emission. The RS waterlogging approach on the soil surface possibly reduces CH 4 emission due to not being buried into the soil. However, evaluation of CH 4 emission by this approach has not been determined. The objective of this study was to examine CH 4 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 CH 4 emissions and CH 4 accumulation for the rice-growing and off-sowing periods. Yield-scaled CH 4 emission was assessed based on total methane emission and rice yield.

Results: The results demonstrated that RS waterlogging reduced CH 4 emission by 16.9% compared to RS incorporation into the soil. During the rice-growing period, total CH 4 emission 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 CH 4 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 CH 4 emission. However, high CH 4 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.

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

The agricultural sector contributes approximately 10–12% of global anthropogenic emissions. Of these emissions, 47% of methane (CH 4) emission has been attributed to agriculture production ( Smith et al., 2007). Rice fields have been considered an important source of atmospheric CH 4, accounting for 15–20% of the global total anthropogenic CH 4 emission ( Sass & Fisher, 1997). Rice straw (RS) plays a vital role in contributing to CH 4 emission from paddy fields. Several field experiments have shown that RS incorporation significantly increases CH 4 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 CH 4 emission ( Oda & Chiem, 2019). Several studies have explored decomposing RS in paddy surface water as an effective pathway to reduce CH 4 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 CH 4 emissions. In addition, Oda & Chiem (2019) suggested that the strategy of decomposing RS on surface water effectively reduces CH 4 emission from the paddies. To the best of our knowledge, CH 4 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 CH 4 emission, and the results found that this approach did decrease the CH 4 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 CH 4 following the guidelines recommended by Minamikawa et al. (2015). The chamber (58 cm in length × 38 cm in wide and 90 cm in height) was equipped with a vent to allow equilibration of the pressure, a thermometer, a sampling port, and a fan to ensure well-mixed air inside the chamber while taking the gas sample. Gas sampling was flushed five times with chamber air before collecting. Gas samples were collected with a propylene syringe 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 CH 4 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 CH 4 emissions were calculated using a trapezoidal integration method with linear interpolation and numerical integration between sampling times. The calculation was done as follows: (i) calculate the daily gas flux by multiplying the daily mean hourly gas flux by 24, (ii) calculate the emission between every two consecutive measurements using the trapezoidal rule, and (iii) sum up the areas of all the trapezoids. Yield-scale CH 4 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 CH 4 emission in rice-growing and off-sowing periods as well as yield-scaled CH 4 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 CH 4 emission using a container experiment. The results showed that the CH 4 emission from waterlogging accounted for 36% of the incorporation treatment during the rice-growing period ( Figure 1a) ( Oda et al., 2020). However, high emissions were found in the off-sowing stage ( Figure 1b). The total CH 4 emissions from the waterlogging and incorporation treatments were 502 ± 111.4 kgCH 4.ha -1.crop -1 and 604 ± 41.9 kgCH 4.ha -1.crop -1, respectively. In general, the magnitude of seasonal CH 4 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 kgCH 4.ha -1.crop -1 ( Oda & Chiem, 2019; Vo et al., 2018). The CH 4 emission for the decomposing RS subject to waterlogging was 16.9% lower than that of the straw in the incorporation approach, even though the total timing of the off-sowing and rice-growing period was 30% longer. For the yield-scaled CH 4 emission from straw, the waterlogging and incorporation models were 0.21 ± 0.02 kg CH 4.kg grain -1 and 0.3 ± 0.08 kg CH 4. kg grain -1, respectively. The difference between yield-scaled CH 4 emissions was insignificant (P>0.05) ( Figure 2).

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

 Yield-scaled CH <sub>4</sub> emission of rice straw (RS) waterlogging and RS incorporation. Emission pattern

CH 4 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 CH 4 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).

CH <sub>4</sub> 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 CH 4 emissions. Modification of rice straw practices is undoubtedly necessary to reduce CH 4 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 CH 4 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 CH 4 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 CH 4 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 CH 4 emission from rice straw waterlogging on the field surface promotes aerobic decomposition, resulting in reducing CH 4 emission.

This study showed the effects of rice straw management on CH 4 emission in the rice-growing period. Total CH 4 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 CH 4 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 CH 4 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 O 2 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, CH 4 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 CH 4 emission during the RS waterlogging is more significant than rice straw incorporation. Rice straw CH 4 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 CH 4 generation.

The development of agricultural technologies to reduce CH 4 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 CH 4 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 CH 4 emission in the long-term run.

 Conclusions

We evaluated effects of RS treatment measures on CH 4 emissions under waterlogging and incorporation. Our results indicated that RS decomposition under the waterlogging approach reduces CH 4 emission compared to the incorporation approach, confirming the feasibility of rice straw waterlogging as a mitigation option for paddy CH 4 emission in the Vietnamese Mekong Delta. However, waterlogging significantly contribute to CH 4 emission during the off-sowing period. Thus, we recommend that reducing CH 4 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 CH 4 emission (mgCH 4.m -2.h -1) and cumulative methane emission (kgCH 4.ha -1.crop -1), and yield-scaled CH 4 emission (kg CH 4 grain -1)).

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

 Acknowledgments

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

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Competing interests: No competing interests were disclosed.

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

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

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

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

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

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

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

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

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

Partly

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

Partly

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

No

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

No

Are the conclusions drawn adequately supported by the results?

No

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

Partly

Reviewer Expertise:

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

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

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

Competing interests: No competing interests were disclosed.

 17 6 2022

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

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

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

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

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

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

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

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

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

Reply: We corrected it through the paper.

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

Best regards,

Authors.

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

Competing interests: No competing interests were disclosed.

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

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

 

Please define "off-sowing periods" in Methods.

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

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

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

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

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

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

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

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

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

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

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

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

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

Yes

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

Yes

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

Partly

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

Yes

Are the conclusions drawn adequately supported by the results?

Yes

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

Partly

Reviewer Expertise:

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

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

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

Competing interests: No competing interests were disclosed.

 17 6 2022

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Best regards,

Authors.