Investment in sewage treatment significantly reduces greenhouse gas emissions of urban rivers.

Urban rivers are increasingly acknowledged as a significant source to the emission of greenhouse gases (GHGs). However, less is known about the coupling effects of investment in sewage treatment on CO2, CH4 and N2O outgassing in urban streams and rivers. Here, by assessing a compiled GHGs dataset with more than 10,000 records of urban rivers from 24 Chinese metropolitan areas, we report the relationships of GHG emissions with the investment in sewage treatment, as well as with some key urban development indicators such population and gross domestic product (GDP). We find high GHGs emissions from these urban rivers, i.e., 238.41 (±391.65) CO2 mmol m-2d-1, 10.99 (±28.56) CH4 mmol m-2d-1 and 0.19 (±0.64) N2O mmol m-2d-1. The emission rates are largely variable with the highest in the economically most developed regions, where rivers receive more sewage treatment effluents and nutrient inflow. There is a clear positive effect of sewage treatment investment on reducing stream CO2 and CH4 emission, indicating a reduction in carbon pool. However, this is less effective in reducing N2O emission from urban rivers, as it is positively and significantly related with total nitrogen level and population within the metropolitan areas. Based on these findings, we postulate implementation of monitoring carbon level in sewage treatment effluents as well as improvement of current best management practices and low impact development in urban areas.