Keywords
Greenhouse gases, Mekong Delta, Methanogenesis inhibition, Rice straw, Flooding, Methane reduction
This article is included in the Agriculture, Food and Nutrition gateway.
Greenhouse gases, Mekong Delta, Methanogenesis inhibition, Rice straw, Flooding, Methane reduction
1) The aim of the study is more clearly described.
2) The information about soil and the amount of the incorporated rice straw are added.
3) The emission was changed as a range to each average of crops.
4) Figure 1 is revised as averages of five years and showed the 95% CI.
See the authors' detailed response to the review by Tran Dang Hoa
See the authors' detailed response to the review by Arika Bridhikitti
See the authors' detailed response to the review by Kazuyuki Yagi
Vietnam is the world’s fifth largest rice producer (FAO 2018). The Mekong Delta produces the half (23.8 million tons) (General Statistics Office of Vietnam 2016). The climate of tropical monsoon (Am) enables high productivity by triple rice cropping (cropping three times a year). Rice paddies are a methane emission source, and the Mekong Delta is a hotspot (Arai et al., 2018; Werner et al., 2016). The high emissions are caused by the rice straw incorporation (Oda & Chiem, 2019). However, the methane emission of triple rice cropping has not been well studied (Vo et al., 2018).
The Mekong’s natural flood of two months (starting from around late September to late October) limits the rice cultivation period. The 1st crop (winter-spring) begins after the natural flood, then after harvesting the rice straw is incorporated into the soil. The 2nd (spring-summer) and the 3rd crop (summer-autumn) follows without interval. Just after the 3rd crop, the natural flood starts so the straw is left on the paddies and decomposes under the floodwater. Then, the 1st crop begins again without incorporation of the straw in the soil (field leveling only), because they are sufficiently decomposed by that time.
Can Tho University (CTU) and the Japan International Research Center for Agricultural Sciences (JIRCAS) conducted joint research and monitored methane emissions in typical triple rice cropping paddies for 5 years (for a total of 15 crops). The present study is a specific analysis of a part of the data set from this project. The results show that the strategy of decomposing rice straw on the surface water effectively reduces methane emission from the paddies.
The observation was conducted on a farmer’s paddies (three fields) managed by the above typical triple-cropping in Thuan Hung village (10°22' N, 105°58' E), Thot Not district, Can Tho city, Vietnam from 2011 to 2016. The soil is alluvium soil (aquic Trapaquepts). Normally, from May to October is the rainy season. The farmer managed the water with continuous flooding using a dike system. The rice variety Jasmine was used for the 1st crop, and OM501 was used for the 2nd and 3rd crop every year. The average number of growth days per crop were 103, 89, and 92, for the 1st, 2nd, and 3rd crops, respectively. The average intervals between the 1st and the 2nd crop and the 2nd and the 3rd crop were 5.6 and 6.6 days, respectively. The average rice straw dry weight per crop were 9.0, 9.3, and 7.4 (Mg ha–1), for the 1st, 2nd, and 3rd crops, respectively. This study was conducted with the approval of the farmer. The details were described previously (Oda & Chiem, 2019).
We used the closed chamber method established by NARO and IRRI (http://globalresearchalliance.org/research/paddy-rice/), and the measurements were taken at 8 a.m. (ca. 90% of the average daily emissions). The details were described previously (Oda & Chiem, 2019). In periods of natural flood, chambers with attached Styrofoam floats were used. Measurements were taken once a week throughout the rice growing stage, but every 3 days for 2 weeks after seeding, heading stage, and around draining.
According to the IPCC guidelines, standard methane emissions over 100 days of continuously flooding rice cropping are 130 kg ha−1 crop−1. Wassmann et al. (1996) reported very high emissions (160–240 kg ha−1 crop−1) from double cropping rice paddies in the Philippines after organic matter incorporation. However, we observed larger emissions (710, 1290, and 1789 (kg ha−1 crop−1), for the 1st, 2nd, and 3rd crops in average, respectively. Vo et al. (2018) measured the same level of emission in the Mekong delta (ca. 900 kg CH4 ha–1 crop–1). The emission level doubled in the 2nd crop, and tripled in the 3rd crop, then reset after the natural flood (Figure 1). Furthermore, the total emissions during the flood period and the 1st crop was lower than that of the 3rd crop (Figure 1). The total emission should be higher than that of the 3rd crop; because, the absence of rice plants doubles the methane emission from the field (Oda & Chiem, 2019). Note, we confirmed that no rice straw (the source of methanogenesis) was lost to the floodwater. Raw results are available as Underlying data (Oda, 2019a).
The previous study (Oda & Chiem, 2019) indicated three types of methane emission patterns during the rice growth period. Generally, the emissions peak at the heading stage due to the methanogenesis substrate provided by the present rice. Another pattern can occur with an additional peak at the early stage of rice growth if organic matter was incorporated beforehand. The third is the pattern in the triple rice cropping. The emission peaks at the early stage of rice growth, then gradually decreases; the peak at the heading stage is undetectable because of the high emission levels. This means the contribution of the rice-derived carbon is small. In the present study, the pattern was the same as the previous study (Oda & Chiem, 2019). The emissions began with irrigation, reached peaks from 0 to 3 weeks after the start of irrigation (see Extended data, Supplemental figure; Oda, 2019b), and gradually decreased, and the peak at the heading stage was undetected. Furthermore, the emissions during the natural flood appeared to be a continuation of the emissions of the 3rd crop (Figure 2).
The total emissions in a crop season doubled in the second crop, tripled in the third crop, and reset after the natural flood. This can be explained by the accumulation of rice residue from the preceding crops, especially by the rice straw incorporated into the soil, because the contribution of the present rice-derived carbon is small (Oda & Chiem, 2019).
The reset of emission levels after the annual flood means that the rice straw is decomposed without methanogenesis in water because the water includes dissolved oxygen. The fact that the emissions under natural flood appeared to be a continuation of the emissions of the 3rd crop suggests that the rice straw on the paddy surface contribute to little methane emission. A portion of emission in the first crop will be caused by incorporation of the remaining rice straw related to the leveling of the field.
Our results indicate that the main cause of the increase in methane emissions was the incorporation of rice straw into the soil. In contrast, decomposing rice straw in paddy surface-water generated less methane. Thus, decomposing rice straw in paddy surface-water is an effective method to reduce methane emissions.
We analyzed the methane emission patterns of triple rice cropping paddies in the Mekong Delta. Methane emissions increased with rice straw incorporation into the soil. The natural flood resulted in decomposition occurring in the water, leading to less methane emission. Therefore, the annual emission pattern suggests that decomposing rice straw in paddy surface-water is an effective method to reduce methane emissions. The development of practical technology to attain this reduction is a subject for a future study.
Figshare: Methane emission from triple cropping rice field. https://doi.org/10.6084/m9.figshare.9757934.v1 (Oda, 2019a).
Figshare: Methane flux of days after transplanting. https://doi.org/10.6084/m9.figshare.9746006.v1 (Oda, 2019b).
Data are available under the terms of the Creative Commons Attribution 4.0 International license (CC-BY 4.0).
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Competing Interests: No competing interests were disclosed.
Reviewer Expertise: Environmental Engineer
Is the work clearly and accurately presented and does it cite the current literature?
No
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?
No
If applicable, is the statistical analysis and its interpretation appropriate?
No
Are all the source data underlying the results available to ensure full reproducibility?
Partly
Are the conclusions drawn adequately supported by the results?
Partly
Competing Interests: No competing interests were disclosed.
Reviewer Expertise: biogeochemistry, soil sciences
Competing Interests: No competing interests were disclosed.
Reviewer Expertise: Environmental Engineer
Competing Interests: No competing interests were disclosed.
Reviewer Expertise: Low carbon rice production
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
Competing Interests: No competing interests were disclosed.
Reviewer Expertise: Low carbon rice production
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?
No
If applicable, is the statistical analysis and its interpretation appropriate?
Not applicable
Are all the source data underlying the results available to ensure full reproducibility?
No
Are the conclusions drawn adequately supported by the results?
Partly
Competing Interests: No competing interests were disclosed.
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