Wastewater treatment plant effluents increase the global warming potential in a subtropical urbanized river

Rivers play a pivotal role in global carbon (C) and nitrogen (N) biogeochemical cycles. Urbanized rivers are significant hotspots of greenhouse gases (GHGs, N₂O, CO₂ and CH₄) emissions. This study examined the GHGs distributions in the Guanxun River, an effluents-receiving subtropical urbanized river, as well as the key environmental factors and processes affecting the pattern and emission characteristics of GHGs. Dissolved N₂O, CO₂, and CH₄ concentrations reached 228.0 nmol L^-1, 0.44 mmol L^-1, and 5.2 μmol L^-1 during the wet period, and 929.8 nmol L^-1, 0.7 mmol L^-1, and 4.6 μmol L^-1 during the dry period, respectively. Effluents inputs increased C and N loadings, reduced C/N ratios, and promoted further methanogenesis and N₂O production dominated by incomplete denitrification after the outfall. Increased urbanization in the far downstream, high hydraulic residence time, low DO and high organic C environment promoted methanogenesis. The strong CH₄ oxidation and methanogenic reactions inhibited by the effluents combined to suppress CH₄ emissions in downstream near the outfall, and the process also contributed to CO₂ production. The carbon fixation downstream from the outfall were inhibited by effluents. Ultimately, it promoted CO₂ emissions downstream from the outfall. The continuous C, N, and chlorine inputs maintained the high saturation and production potential of GHGs, and altered microbial community structure and functional genes abundance. Ultimately, the global warming potential downstream increased by 186 % and 84 % during wet and dry periods on the 20-year scale, and increased by 91 % and 49 % during wet and dry periods on the 100-year scale, respectively, compared with upstream from the outfall. In urbanized rivers with sufficient C and N source supply from WWTP effluents, the large effluent equivalently transformed the natural water within the channel into a subsequent "reactor". Furthermore, the IPCC recommended EF_5r values appear to underestimate the N₂O emission potential of urbanized rivers with high pollution loading that receiving WWTP effluents. The findings of this study might aid the development of effective strategies for mitigating global climate change.