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 (CH ₄ ) and nitrous oxide (N ₂ O) 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 CH ₄ emissions by 53%, increased N ₂ O emissions by 105%, and decreased yield by 3.6%. Importantly, N ₂ O emissions were low, contributing, on average, 12% to the combined global warming potential (GWP; CH ₄ + N ₂ O). As a result, non-continuous flooding reduced GWP (-44%) and yield-scaled GWP (-42%). However, non-continuous flooding practices stimulated N ₂ O emissions to a greater degree in soils with high organic carbon or with manure additions. The reduction in CH ₄ 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.