Site-specific feasibility of alternate wetting and drying as a greenhouse gas mitigation option in irrigated rice fields in Southeast Asia: a synthesis
This study comprises a comprehensive assessment, integration, and synthesis of data gathered from a 3-year field experiment conducted at four sites in Southeast Asia, namely Hue, Vietnam; Jakenan, Indonesia; Prachin Buri, Thailand; and Muñoz, Philippines, to assess the site-specific feasibility of alternate wetting and drying (AWD) as a greenhouse gas (GHG) mitigation option in irrigated rice fields. AWD effectively reduced water use compared to continuous flooding (CF) but did not significantly reduce rice grain yield and soil carbon content in all sites. Methane (CH4) emissions varied significantly among sites and seasons as affected by soil properties and water management. AWD reduced CH4 emissions relative to CF by 151 (25%), 166 (37%), 9 (31%), and 22 (32%) kg CH4 ha−1 season−1 in Hue, Jakenan, Prachin Buri, and Muñoz, respectively. In Prachin Buri and Muñoz, AWD reduced CH4 emissions only during the dry season. Site-specific CH4 emission factors (EFs) ranged 0.13–4.50 and 0.08–4.88 kg CH4 ha−1 d−1 under CF and AWD, respectively. The mean AWD scaling factors (SFs) was 0.69 (95% confidence interval: 0.61–0.77), which is slightly higher than the Intergovernmental Panel on Climate Change (IPCC)’ SF for multiple aeration of 0.52 (error range: 0.41–0.66). Significant reductions in the global warming potential (GWP) of CH4+nitrous oxide (N2O) by AWD were observed in Hue and Jakenan (27.8 and 36.1%, respectively), where the contributions of N2O to the total GWP were only 0.8 and 3.5%, respectively. In Muñoz, however, CH4 emission reduction through AWD was offset by the increase in N2O emissions. The results indicate that the IPCC’s SF for multiple aeration may only be applied to irrigated rice fields where surface water level is controllable for a substantial period. This study underscores the importance of practical feasibility and appropriate timing of water management in successful GHG reductions by AWD.