Intense methane ebullition from urban inland waters and its significant contribution to greenhouse gas emissions.

The evasions of methane (CH4) and carbon dioxide (CO2) from inland waters represent substantial fluxes of greenhouse gases into the atmosphere, offsetting a large part of the continental carbon sink. However, the CH4 and CO2 emissions from urban inland waters are less constrained. In particular, ebullitive CH4 emissions from these waters are poorly understood. Here, we measured the concentrations and fluxes of CH4 and CO2 in rivers and lakes in the megacity of Beijing, China, between 2018 and 2019. The CH4 concentration ranged from 0.08 to 70.2 µmol L-1 with an average of 2.5 ± 5.9 µmol L-1. The average CH4 ebullition was 11.3 ± 30.4 mmol m-2 d-1 and was approximately 6 times higher than the global average. The average total CH4 flux (14.2 ± 35.1 mmol m-2 d-1) was 3 times higher than the global average, with ebullition accounting for 80% of the flux. The high surface water CH4 concentrations and ebullitive fluxes were caused by high sediment organic carbon/dissolved organic carbon contents, high aquatic primary productivity and shallow water depths in the urban inland waters. The CH4 emissions accounted for 20% of CO2 emissions in terms of the carbon release and were 1.7 times higher in terms of CO2 equivalent emissions from Beijing inland waters. Furthermore, the CH4 ebullition and its contribution to the total carbon gas emissions increased exponentially with the water temperature, suggesting a positive feedback probably occurs between the greenhouse gas emissions from urban inland waters and climate warming. This study confirms the major role of CH4 ebullition from urban inland waters in the global carbon budget under the rapid progress of global urbanization.