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Biochar is recommended as soil amendment to CH4 emissions from rice growing. However, CH4 mitigation mechanisms are not well known. This study aimed to evaluate the changes of CH4 production, oxidation and emission of rice cultivation soil that was amended by mangrove biochar to explain the mitigation way. Biochar at rate equivalent to 10 t ha-1 was thoroughly mixed with clay loam soil within the 0.32 m3 bucket on 16th d before rice was sown under greenhouse condition. The redox potential (Eh), dissolved organic carbon (DOC), sulfate (SO42-) and nitrate (NO3-) of soil and CH4 emission were measured throughout cultivation season, while abundances of methanogens and methanotrophs and activities of CH4 production and oxidation were determined from soil that collected in two critical stages (tillering and heading) of rice growth. The results show that biochar enhanced soil oxidation capacity through increasing soil Eh (ranging of 11-26 mV) and major electron acceptors (average of 77.3 % in SO42- and 42.1 % in NO3-) throughout rice growth period. It also enhanced soil DOC in the most stages of rice cultivation. These effects did not influence the significant change of CH4 producer (methanogenic archaea) and CH4 production activity as compared to no amendment soil. A high porosity and surface area of mangrove biochar stimulated CH4 consumer (methanotrophic bacteria), resulting in enhanced CH4 oxidation activity by 11.8 % in tillering stage and 34.4 % in heading stage. This greater CH4 oxidation activity influenced the significant reduction of seasonal CH4 emission by 24.5 %. A major factor of CH4 emissions mitigation from rice cultivation by biochar amendment is the increment of CH4 oxidation particularly during heading stage. [ABSTRACT FROM AUTHOR]

Biochar application and alternate wetting and drying (AWD) are emerging as promising technologies recommended for reducing CH4 emissions and water consumption in rice cultivation. In this study, we hypothesized that both technologies could be practiced in combination and this could further reduce CH4 emissions and water consumption when compared to practicing alone. The effects of biochar application and its co-application with chemical fertilizer or compost under conventional or AWD water management on CH4 emissions, productivity of rice, water use, and SOC stock, as well as cost and income were investigated. The experiment was carried out in an irrigated paddy field in the central plain of Thailand during both in the wet and dry seasons. Relative to control (CT), biochar application (BI), its co-application with compost (BC) or chemical fertilizer (BF) reduced seasonal CH4 emissions by 40.6%, 29.5%, and 12.3%, respectively. BI and BC significantly (p < 0.05) reduced grain yield by 19.9% and 10.8%, respectively, while BF significantly increased grain yield by 3.70%. In addition, BI, BC, and BF significantly enhanced soil organic carbon (SOC) stock by 21.2%, 21.4%, and 18.3%, respectively. Compared to the CT, higher production costs were found in BC and BF, but the farmer's net incomes were also higher in BF because of its higher grain yield. On the other hand, water management in all amendment treatments under AWD was resulted in the reduction of CH4 emissions by the average of 18.8% as compared to the conventional system. AWD decreased rice yield by an average of 2.29%. It significantly reduced irrigation water use by an average of 11.9%, resulting in reducing production cost for water pumping. The results show that the practice that combined biochar application, AWD and chemical fertilizer are feasible for CH4 emission mitigation, SOC stock increase and irrigation water saving without significant effects on yield and farmer income. [ABSTRACT FROM AUTHOR]

Paddy fields are considered as one of the most important sources of anthropogenic methane (CH4) and nitrous oxide (N2O) emissions. While several technical options have been proposed to reduce these emissions, gaps in data and information based on application of these options in the field are a key barrier to scaling-up. To address these gaps, we conducted a review of literature to analyze the potential of technical options in Southeast Asia (SEA). Using screening criteria based on reliability of experimental data, 31 region-specific cases were selected for the analysis. A meta-analysis indicated that water management options, including single and multiple drainage approaches such as alternative wetting and drying (AWD), significantly reduced CH4 emissions by 35% as a mean effect size (95% confidential interval: 41–29%), as well as the combined effects of CH4+N2O (net GWP) by 29% (36–23%). The effect on reducing CH4 emissions in the dry season was significantly larger than that in the wet season. Application of biochar reduced both CH4 and N2O emissions by 20% (40% to −7%), while significantly increased rice yield by 28% (8–52%). Other options such as removal of rice straw from the previous crop, composting rice straw and manure, application of sulfate-containing fertilizer, and soil drying in the fallow season also have recognized potential to reduce emissions but require further data and consideration of possible trade-offs. Based on the analysis of mitigation potential, promising technical options were assessed by considering together with constraints and additional co-benefits in order to provide a useful guide for policy makers and rice value chain operators in SEA countries for adopting mitigation options in rice cultivation to tackle climate change and enhance agriculture sector sustainability. [ABSTRACT FROM AUTHOR]

Alternate wetting and drying (AWD) is a water-saving irrigation technique in a paddy field that can reduce the emission of methane, a potent greenhouse gas (GHG). It is being adopted to Asian countries, but different results are reported in literatures on methane, nitrous oxide emission, and rice productivity under AWD. Therefore, the local feasibility needs to be investigated before its adoption by farmers. The current study carried out a 3-year experiment in an acid sulfate paddy field in Prachin Buri, Thailand. During five crops (3 dry and 2 wet seasons), three treatments of water management were compared: continuous flooding (CF), flooding whenever surface water level declined to 15 cm below the soil surface (AWD), and site-specific AWD (AWDS) that weakened the criteria of soil drying (AWDS). Methane and nitrous oxide emissions were measured by a closed chamber method. Rice grain yield did not significantly (p < 0.05) differ among the three treatments. The amount of total water use (irrigation + rainfall) was significantly reduced by AWD (by 42%) and AWDS (by 34%) compared to CF. There was a significant effect of treatment on the seasonal total methane emission; the mean methane emission in AWD was 49% smaller than that in CF. The seasonal total nitrous oxide emission and the global warming potential (GWP) of methane and nitrous oxide did not differ among treatments. The contribution of nitrous oxide to the GWP ranged 39–62% among three treatments in dry season whereas 3–13% in wet season. The results indicate that AWD is feasible in terms of GHG emission mitigation, rice productivity, and water saving in this site, especially in dry season. [ABSTRACT FROM PUBLISHER]