Elevated nitrogen loadings facilitate carbon dioxide emissions from urban inland waters

Carbon dioxide (CO₂) production and emissions from inland waters play considerable roles in global atmospheric CO₂ 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 CO₂ emissions. Here, we investigated the spatiotemporal patterns of partial pressure of CO₂ (pCO₂) and CO₂ fluxes (FCO₂) in typical urban inland waters in Tianjin, China. Our observation indicated that pCO₂ 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 pCO₂ and FCO₂ emphasized that the water environmental conditions and anthropogenic activities jointly adjusted primary productivity and biological respiration of inland waters. Meanwhile, statistically positive correlations between pCO₂/FCO₂ and NH₄⁺-N/NO₃^--N (p < 0.05) suggested that nitrogen biogeochemical processes, especially the nitrification, played a dominant role in CO₂ emissions attributing to the water acidification that stimulated CO₂ production and emissions. Except for slight CO₂ sinks in waters with low organic contents, the total CO₂ emissions from the urban surface waters of Tianjin were remarkable (286.8 Gg yr^-1). The results emphasized that the reductions of nitrogen loadings, sewage draining waters, and agricultural pollution could alleviate CO₂ emissions from urban inland waters.