Abstract
Global rice cultivation is estimated to account for 2.5% of current anthropogenic warming because of emissions of methane (CH(4)), a short-lived greenhouse gas. This estimate assumes a widespread prevalence of continuous flooding of most rice fields and hence does not include emissions of nitrous oxide (N(2)O), a long-lived greenhouse gas. Based on the belief that minimizing CH(4) from rice cultivation is always climate beneficial, current mitigation policies promote increased use of intermittent flooding. However, results from five intermittently flooded rice farms across three agroecological regions in India indicate that N(2)O emissions per hectare can be three times higher (33 kg-N(2)O⋅ha(-1)⋅season(-1)) than the maximum previously reported. Correlations between N(2)O emissions and management parameters suggest that N(2)O emissions from rice across the Indian subcontinent might be 30-45 times higher under intensified use of intermittent flooding than under continuous flooding. Our data further indicate that comanagement of water with inorganic nitrogen and/or organic matter inputs can decrease climate impacts caused by greenhouse gas emissions up to 90% and nitrogen management might not be central to N(2)O reduction. An understanding of climate benefits/drawbacks over time of different flooding regimes because of differences in N(2)O and CH(4) emissions can help select the most climate-friendly water management regimes for a given area. Region-specific studies of rice farming practices that map flooding regimes and measure effects of multiple comanaged variables on N(2)O and CH(4) emissions are necessary to determine and minimize the climate impacts of rice cultivation over both the short term and long term.