Methane and nitrous oxide concentrations and fluxes from heavily polluted urban streams: Comprehensive influence of pollution and restoration.

Streams draining urban areas are usually regarded as hotspots of methane (CH) and nitrous oxide (NO) emissions. However, little is known about the coupling effects of watershed pollution and restoration on CH and NO emission dynamics in heavily polluted urban streams. This study investigated the CH and NO concentrations and fluxes in six streams that used to be heavily polluted but have undergone different watershed restorations in Southwest China, to explore the comprehensive influences of pollution and restoration. CH and NO concentrations in the six urban streams ranged from 0.12 to 21.32 μmol L and from 0.03 to 2.27 μmol L, respectively. The calculated diffusive fluxes of CH and NO were averaged of 7.65 ± 9.20 mmol m d and 0.73 ± 0.83 mmol m d, much higher than those in most previous reports. The heavily polluted streams with non-restoration had 7.2 and 7.8 times CH and NO concentrations higher than those in the fully restored streams, respectively. Particularly, CH and NO fluxes in the fully restored streams were 90% less likely than those found in the unrestored ones. This result highlighted that heavily polluted urban streams with high pollution loadings were indeed hotspots of CH and NO emissions throughout the year, while comprehensive restoration can effectively weaken their emission intensity. Sewage interception and nutrient removal, especially N loadings reduction, were effective measures for regulating the dynamics of CH and NO emissions from the heavily polluted streams. Based on global and regional integration, it further elucidated that increasing environment investments could significantly improve water quality and mitigate CH and NO emissions in polluted urban streams. Overall, our study emphasized that although urbanization could inevitably strengthen riverine CH and NO emissions, effective eco-restoration can mitigate the crisis of riverine greenhouse gas emissions.