Elevated nitrogen loadings facilitate carbon dioxide emissions from urban inland waters.

Carbon dioxide (CO) production and emissions from inland waters play considerable roles in global atmospheric CO sources, while there are still uncertainties regarding notable nutrient inputs and anthropogenic activities. Urban inland waters, with frequently anthropogenic modifications and severely nitrogen loadings, were hotspots for CO emissions. Here, we investigated the spatiotemporal patterns of partial pressure of CO (pCO) and CO fluxes (FCO) in typical urban inland waters in Tianjin, China. Our observation indicated that pCO values were oversaturated in highly polluted waters, particularly in sewage rivers and urban rivers, exhibiting approximately 9 times higher than the atmosphere equilibrium concentration during sampling campaigns. Obviously, the spatiotemporal distributions of pCO and FCO emphasized that the water environmental conditions and anthropogenic activities jointly adjusted primary productivity and biological respiration of inland waters. Meanwhile, statistically positive correlations between pCO/FCO and NH-N/NO-N (p < 0.05) suggested that nitrogen biogeochemical processes, especially the nitrification, played a dominant role in CO emissions attributing to the water acidification that stimulated CO production and emissions. Except for slight CO sinks in waters with low organic contents, the total CO emissions from the urban surface waters of Tianjin were remarkable (286.8 Gg yr). The results emphasized that the reductions of nitrogen loadings, sewage draining waters, and agricultural pollution could alleviate CO emissions from urban inland waters.