Water management to mitigate the global warming potential of rice systems: A global meta-analysis

Rice is a main staple food for roughly half of the world's population, but rice agriculture is also a main source of anthropogenic greenhouse gas (GHG) emissions. Many studies have reported that water management (e.g. alternate wetting and drying, intermittent irrigation, mid-season drain, aerobic rice) affects rice yields and methane (CH4) and nitrous oxide (N2O) emissions from rice paddies. However, these studies span a variety of practices and vary in experimental design and results, making it difficult to determine their global response from individual experiments. Here we conducted a meta-analysis using 201 paired observations from 52 studies to assess the effects of water management practices on GHG emissions and rice yield. Overall, compared to continuous flooding, non-continuous flooding practices reduced CH4 emissions by 53%, increased N2O emissions by 105%, and decreased yield by 3.6%. Importantly, N2O emissions were low, contributing, on average, 12% to the combined global warming potential (GWP; CH4 + N2O). As a result, non-continuous flooding reduced GWP (-44%) and yield-scaled GWP (-42%). However, non-continuous flooding practices stimulated N2O emissions to a greater degree in soils with high organic carbon or with manure additions. The reduction in CH4 emissions increased with the number of drying events, soil drying severity, and the number of unflooded days. Currently, Intergovernmental Panel on Climate Change (IPCC) scaling factors for single and multiple (>= 2) drying events are 0.6 and 0.52. Based on this analysis using actual side-by-side field studies, we suggest changing these to 0.67 for a single event and 0.36 for multiple events.