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

Carbon dioxide (CO ₂ ) production and emissions from inland waters play considerable roles in global atmospheric CO ₂ 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 ₂ emissions. Here, we investigated the spatiotemporal patterns of partial pressure of CO ₂ (pCO ₂ ) and CO ₂ fluxes (FCO ₂ ) in typical urban inland waters in Tianjin, China. Our observation indicated that pCO ₂ 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 ₂ and FCO ₂ 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 ₂ /FCO ₂ and NH ₄ ⁺ -N/NO ₃ ^- -N (p < 0.05) suggested that nitrogen biogeochemical processes, especially the nitrification, played a dominant role in CO ₂ emissions attributing to the water acidification that stimulated CO ₂ production and emissions. Except for slight CO ₂ sinks in waters with low organic contents, the total CO ₂ 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 CO ₂ emissions from urban inland waters.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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