Greenhouse Gases from Irrigated Rice Systems under Varying Severity of Alternate-Wetting and Drying Irrigation

Rice (Oryza sativa L.) is normally grown under flooded conditions and is a significant source of methane (CH4). Alternate wetting and drying (AWD) is one practice which has shown promise to reduce CH4 emissions and global warming potential (GWP). Under AWD, the soil is allowed to dry periodically during the growing season. In this 2-yr field study, three different severities of drying were compared to a continuously flooded condition to quantify effects on rice yields, greenhouse gas emissions, GWP and yield-scaled GWP (GWP(Y)). The AWD treatments in order of increasing drying severity were: Safe-AWD (AWD(S)) where plots were reflooded when the perched water table fell 15 cm below the soil surface (volumetric water content of 41 to 44%); and AWD(35) and AWD(25 )where plots were reflooded when the soil volumetric water content reached approximately 35 and 25%, respectively. Each of these treatments received two drying cycles (all occurring between 45 d after planting and heading). Grain yields and cumulative N2O emissions (close to zero) did not vary significantly among treatments. The AWD(S) reduced CH4 emissions by 41% and the AWD(35) and AWD(25 )by 56 to 73% and 60 to 67%, respectively. Since only CH4 differed between treatments, AWD reduced GWP and GWP(y) by the same relative amount as CH4. Increasing drying severity reduced CH4, GWP and GWP y emissions up to a point (AWD(35)) but continued drying (AWD(25)) did not further reduce CH4 emissions. Given the high early season CH 4 fluxes, drying earlier may result in greater reductions of CH4 in wet seeded rice systems but this requires further study as there may be negative effects such as increased N2O emissions.