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

Core Ideas Rice cultivation under flooded conditions is a significant source of methane. Increasing drying severity decreased methane by 41 to 71% compared to flooded conditions. Drying beyond a certain level did not result in further methane emission reductions. Co‐management of nitrogen and drying time ensured that N 2 O emissions remained low. Increasing drying severity resulted in no reduction in grain yields. Rice ( Oryza sativa L.) is normally grown under flooded conditions and is a significant source of methane (CH 4 ). Alternate wetting and drying (AWD) is one practice which has shown promise to reduce CH 4 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 N 2 O emissions (close to zero) did not vary significantly among treatments. The AWD S reduced CH 4 emissions by 41% and the AWD 35 and AWD 25 by 56 to 73% and 60 to 67%, respectively. Since only CH 4 differed between treatments, AWD reduced GWP and GWP Y by the same relative amount as CH 4 . Increasing drying severity reduced CH 4 , GWP and GWP Y emissions up to a point (AWD 35 ) but continued drying (AWD 25 ) did not further reduce CH 4 emissions. Given the high early season CH 4 fluxes, drying earlier may result in greater reductions of CH 4 in wet seeded rice systems but this requires further study as there may be negative effects such as increased N 2 O emissions.