High Methane Emissions Largely Attributed to Ebullitive Fluxes from a Subtropical River Draining a Rice Paddy Watershed in China.

Abstract

Rivers are of increasing concern as sources of atmospheric methane (CH4), while estimates of global CH4 emissions from rivers are poorly constrained due to a lack of representative measurements in tropical and subtropical latitudes. Measurements of complete CH4 flux components from subtropical rivers draining agricultural watersheds are particularly important since these rivers are subject to large organic and nutrient loads. Two-year measurements of CH4 fluxes were taken to assess the magnitude of CH4 emissions from the Lixiahe River (a tributary of the Grand Canal) draining a subtropical rice paddy watershed in China. Over the two-year period, annual CH4 emissions averaged 29.52 mmol m-2 d-1, amounting to 10.78 mol m-2 yr-1, making the river a strong source of atmospheric CH4. The CH4 emissions from rivers during the rice-growing season (June-October) accounted for approximately 70% of the annual total, with flux rates at 1-2 orders of magnitude greater than those for rice paddies in this area. Ebullition contributed to 44-56% of the overall CH4 emissions from the rivers and dominated the emission pathways during the summer months. Our data highlight that rivers draining agricultural watersheds may constitute a larger component of anthropogenic CH4 emissions than is currently documented in China.