Greenhouse gas emissions from urban river waters of China's major cities

• GHG emissions from urban river waters in China's 23 major cities are elevated. • CO 2 emission in the urban river waters is highest, followed by CH 4 and N 2 O. • Nutrients and organic carbon from WWTP effluents are indicative of GHG emissions. • Significantly higher GHG emission occur in the urban rivers during summertime. Urban rivers are emerging as potent emitters of greenhouse gases (GHGs). However, the magnitude and drivers of GHG emissions from urban rivers are not well understood at a national level. Using a quantitative synthesis of 4633 data records collected from urban river waters of 23 major Chinese cities, this study delved into a systematic analysis of GHG emissions from urban rivers across China. The study showed that GHG emissions in these urban rivers varied spatially and temporally, with the highest carbon emission (CO 2 + CH 4 ) from the rivers draining large metropolitan areas and during the summertime. CO 2 , CH 4 , and N 2 O concentrations in the urban river waters ranged from 22.53 to 1052.20 (188.41 ± 223.10) μmolL − 1 , 0.02 to 21.32 (1.35 ± 1.59) μmolL − 1 , and 0.01 to 16.81 (0.21 ± 0.72) μmolL − 1 , respectively, with an average flux of 148.98 ± 245.30 mmolm − 2 d − 1 for CO 2 , 9.41 ± 25.08 mmolm − 2 d − 1 for CH 4 , and 0.19 ± 0.65 mmolm − 2 d − 1 for N 2 O. Urban rivers also received large domestic sewage with high nutrient and carbon loadings due to huge populations, indicating the effect of wastewater treatment plant effluent and urban runoff on GHG production. These findings imply that direct GHG contributions from rivers in urban areas should be accurately quantified and consequently be included in GHG inventories of riverine GHG emissions.