Effects of alternate wetting and drying technique on greenhouse gas emissions from irrigated rice paddy in Central Luzon, Philippines

Alternate wetting and drying (AWD) technique has been widely reported to reduce methane (CH4) emission from irrigated rice paddies. However, little is known about its feasibility in an environment that has distinct weather conditions involving tropical wet season (WS). To investigate the AWD's feasibility in terms of reducing greenhouse gas (GHG) emissions both in dry season (DS) and WS, 3-year field experiments were conducted in Central Luzon, Philippines. Three treatments of water management were continuous flooding (CF), flooding when surface water level naturally declines to 15 cm below the soil surface (AWD), and site-specific AWD that modified the criteria of soil drying (AWDS). Methane and nitrous oxide (N2O) fluxes were measured using a closed chamber method, and the global warming potential (GWP) of the two GHGs was calculated. Rice grain yield did not significantly differ among treatments. In accordance with the previous findings, the seasonal total CH4 emission was significantly greater in WS than in DS. The effect of treatment was significant, but the reduction rate by AWD was just 1.7% compared to CF. The seasonal total N2O emission was significantly affected by cropping season and treatment. The AWD increased the N2O emission by 97%, especially in DS. The resultant GWP did not significantly differ among three treatments. The results indicate that the AWD and AWDS with the current settings were insufficient to reduce the annual GHG emissions in this site. However, fragmentary results obtained in the last DS suggest that an earlier rice residue incorporation and keeping dry soil conditions in the preceding fallow season is effective in reducing CH4 emission in combination with an earlier implementation of AWD.