Testing the reduction of methane emissions from alternate wetting and drying in rice fields: two years of eddy covariance measurements from Arkansas

Rice agriculture contributes to significant emissions of methane (CH4), a potent greenhouse gas. To mitigate these emissions, carbon offset registries are developing protocols that encourage the alternate wetting and drying (AWD) irrigation practice. In AWD irrigation is temporarily suspended, resulting in drying of the soil and a corresponding reduction in anaerobic decomposition. The result of AWD is to reduce CH4 emissions and conserve water in rice fields, relative to continuous flooding (CF) irrigation. Existing efforts to quantify CH4 emissions from rice fields have largely focused on small plot trials and/or chamber based techniques. To test whether these findings translate to the commercial scale, we used the eddy covariance method to asses CH4 emissions during the 2015 and 2016 growing seasons from two co-located rice fields in Arkansas—the leading rice-producing state in the U.S. One field was subject to AWD and the other field was continuously flooded. Eddy covariance measurements were supplemented by sampling of biometeorological variables, crop development metrics, and soil characteristics. In 2015 we found that AWD significantly reduced CH4 emissions without adversely affecting crop yield. Seasonal cumulative carbon losses due to CH4 emission were 30.3 ± 6.3 and 141.9 ± 8.6 kg CH4-C ha-1 for the AWD and CF irrigation treatments, respectively. In a before-after-control-impact analysis, in which the sites' field conditions and soils were considered, seasonal CH4 emissions were 36-51% less in the AWD field, compared to the CF field. Our seasonal CH4 emissions were less than current estimates of U.S. methane emissions from rice agriculture. The ongoing 2016 growing season helps test the influence of field characteristics on CH4 emissions by switching the irrigation treatments assigned to each field.