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 (CH4) and nitrous oxide (N2O) emissions. However, little is known about the coupling effects of watershed pollution and restoration on CH4 and N2O emission dynamics in heavily polluted urban streams. This study investigated the CH4 and N2O 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. CH4 and N2O concentrations in the six urban streams ranged from 0.12 to 21.32 µmol L-1 and from 0.03 to 2.27 µmol L-1, respectively. The calculated diffusive fluxes of CH4 and N2O were averaged of 7.65 ± 9.20 mmol m-2 d-1 and 0.73 ± 0.83 mmol m-2 d-1, much higher than those in most previous reports. The heavily polluted streams with non-restoration had 7.2 and 7.8 times CH4 and N2O concentrations higher than those in the fully restored streams, respectively. Particularly, CH4 and N2O 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 CH4 and N2O 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 CH4 and N2O 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 CH4 and N2O emissions in polluted urban streams. Overall, our study emphasized that although urbanization could inevitably strengthen riverine CH4 and N2O emissions, effective eco-restoration can mitigate the crisis of riverine greenhouse gas emissions.

pCO2 and CO2 evasion from two small suburban rivers: Implications of the watershed urbanization process.

Rivers are widely reported as CO2-emitting hotpots and are attracting increasing attention worldwide. However, less attention has been given to the CO2 emission from the suburban rivers which are experiencing rapid watershed urbanization and increasing anthropogenic stress. Here, two small suburban rivers in Southwest China were studied, and seasonal sampling campaigns with high spatial resolution were carried out to explore the characterization of partial pressure (pCO2) and CO2 efflux and their possible controls. The results showed that, the pCO2 and estimated CO2 fluxes from the two suburban rivers ranged from 37 to 6466 µatm (mean of 1293 ± 1126 µatm) and -72-1569 mmol·m-2·d-1 (mean of 185 ± 240 mmol·m-2·d-1), respectively. And, both of them exhibited disproportionately high variability and acted as strong CO2 emitters to the atmosphere. The pCO2 in the two suburban rivers showed significant spatial variability, with urban sections having 2-2.5 times higher values than exurban sections, and, the urban land use proportion in the basins accounted for 35%-67% of such spatial variation in pCO2. The sewage-dominated urban tributaries had much higher pCO2 and acted as an obvious exciter to the high pCO2 in urban sections of suburban rivers. Carbon and nutrients concentrations also accounted for the spatial variation in pCO2 and fCO2 in the two suburban rivers, and acted as good indicators. The seasonal variation in pCO2, with the highest values in autumn and lowest values in spring, was controlled by the precipitation dilution effect and seasonal temperature as well as the boosted primary production at several urban sites. We highlighted that small suburban rivers showed disproportionally high spatial variability in pCO2 and CO2 fluxes in their limited basin areas due to the development of urbanization, and could be used as a good model for studying the complex impacts of anthropogenic disturbances on river carbon biogeochemical processes.