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.