Impacts of alternate wetting and drying on greenhouse gas emission from paddy field in Central Vietnam

Vietnam is the world's fifth largest rice producing country. Since methane (CH4), a potent greenhouse gas (GHG), emission from the rice cultivation accounts for 14.6% of the national anthropogenic GHG emission, developing and disseminating mitigation options are the urgent need. Alternate wetting and drying (AWD) is the irrigation technique, in which a paddy field encompasses several soil-drying phases during the growth period, thereby reducing the CH4 emission. However, field trials of the AWD's feasibility in Central Vietnam are limited so far. We therefore carried out a 3-year experiment in a farmer's field both in winter-spring season and summer-autumn season. CH4 and nitrous oxide (N2O) emissions were compared among the three treatments of water management: continuous flooding (CF), AWD, and site-specific AWD (AWDS) that changed the degree of soil drying depending on the growth stage. The total water use including irrigation and rainfall was significantly (p<0.05) reduced by AWD (by 15%) and AWDS (by 14%) compared to CF, but rice grain yield did not differ among the three treatments. The seasonal cumulative CH4 emission was significantly reduced by AWD (26%) and AWDS (26%) compared to CF, whereas the seasonal cumulative N2O emission did not differ among treatments. The resultant global warming potentials (GWPs) of CH4 and N2O under AWD and AWDS were 26% and 29% smaller than that under CF, respectively. The GWP of N2O was only 0.8% of the total GWP of CH4+N2O. The yield-scaled GWP and water productivity (i.e., the ratio of grain yield to water use) were also improved by AWD and AWDS. No significant differences in the measured items between AWD and AWDS were attributed to similar variation patterns in the surface water level. The results confirm the AWD's performance as a mitigation option for paddy GHG emission in Central Vietnam.