The Impacts of Nitrogen Pollution and Urbanization on the Carbon Dioxide Emission from Sewage-Draining River Networks

Carbon dioxide (CO2) emissions from river water have sparked worldwide concerns due to supersaturate CO2 levels in the majority of global rivers, while the knowledge on the associations among nitrogen pollution, urbanization, and CO2 emissions is still limited. In this study, the CO2 partial pressure (pCO(2)), carbon and nitrogen species, and water parameters in sewage-draining river networks were investigated. Extremely high pCO(2) levels were observed in sewage and drainage river waters, such as Longfeng River, Beijing-drainage River, and Beitang-drainage River, which were approximately 4 times higher than the averaged pCO(2) in worldwide rivers. Correlations of carbon/nitrogen species and pCO(2) indicated that carbon dioxide in rural rivers and sewage waters primarily originated from soil aeration zones and biological processes of organic carbon/nitrogen input from drainage waters, while that in urban rivers and lakes was mainly dominated by organic matter degradation and biological respiration. Enhanced internal primary productivity played critical roles in absorbing CO2 by photosynthesis in some unsaturated pCO(2) sampling sites. Additionally, higher pCO(2) levels have been observed with higher NH4+-N and lower DO. CO2 fluxes in sewage waters exhibited extremely high levels compared with those of natural rivers. The results could provide implications for assessing CO2 emissions in diverse waters and fulfilling water management polices when considering water contamination under intense anthropogenic activities.