Using Alternative Water Management to Reduce Greenhouse Gas Emissions and Maintain Yields in California Rice Systems.

Rice production is a significant anthropogenic source of methane (CH₄), a potent greenhouse gas, which is produced in the anaerobic conditions associated with continuously flooded rice fields. Studies in some rice-growing regions have shown that the alternation of wet (flooded) and dry (drained) growing conditions (referred to as AWD) has the potential to mitigate methane emissions, but this management practice has yet to be evaluated in California rice systems.  In this study, nitrous oxide (N₂O) and CH₄ emissions, rice grain yield, and N response from three water management treatments were compared: water-seeded rice grown with conventional continuous flooding (WS-C); water-seeded rice grown under flooded conditions until canopy closure and then flush-irrigated (WS-AWD); and drill-seeded rice grown with flush-irrigation (DS-AWD). Methane emissions were the highest in the WS-C treatment and the lowest in the DS-AWD treatment. In contrast, N₂O emissions were the highest in the DS-AWD treatment and did not differ significantly between the WS-AWD and the WS-C treatments, although N₂O emissions were < 1% of the total Global Warming Potential (GWP) during the growing season. The total GWP (kg CO2 eq ha^-1 season^-1) was therefore lowest in the DS-AWD treatment and highest in the WS-C treatment.  Rice grain yields and response to N fertilizer were not significantly different among the three treatments. Alternative water management therefore represents a viable alternative to allow California rice growers to maintain rice grain yields with similar N fertilization rates while significantly decreasing greenhouse gas emissions.