Denitrification regulates spatiotemporal pattern of N2O emission in an interconnected urban river-lake network

Urban rivers are hotspots of N2O production and emission. Interconnected river-lake networks are constructed to improve the water quality and hydrodynamic conditions of urban rivers in many cities of China. However, the impact of the river-lake connectivity project on N2O production and emission remains unclear. This study investigated dissolved N2O and emission of the river-lake network in Wuhan City, China from March 2021 to December 2021. The results showed that river-lake connection greatly decreased riverine Nitrogen (N) concentration and increased dissolved oxygen (DO) concentration compare to traditional urban rivers. N2O emissions from the urban river interconnected with lakes (LUR: 67.3 ± 92.6 μmol/m2/d) were much lower than those from the traditional urban rivers (UR: 467.3 ± 1075.7 μmol/m2/d) and agricultural rivers (AR: 20.4 ± 15.3μmol/m2/d). Regression tree analysis suggested that the N2O concentrations were extremely high when hypoxia exists (DO < 1.6 mg/L), and TDN was the primary factor regulating N2O concentrations when hypoxia does not occur. Thus, we ascribe the low N2O emission in the LUR and AR to the lower N contents and higher DO concentrations. The microbial process of N2O production and consumption were quantitatively estimated by isotopic models. The mean proportion of denitrification derived N2O (fbD) was 63.5 %, 55.6 %, 42.3 % and 42.7 % in the UR, LUR, lakes and AR, suggested denitrification dominated N2O production in the urban rivers, but nitrification dominated N2O production in the lakes and AR. The positive correlation between logN2O and fbD suggested that denitrification is the key process to regulate the N2O production and emission. The abundance of denitrification genes (nirS and nirK) was much higher than that of nitrification genes (amoA and amoB), also evidenced that denitrification was the main N2O source. Therefore, river-lake interconnected projects changed the nutrients level and hypoxic condition, leading to the inhibition of denitrification and nitrification, and ultimately resulting in a decrease of N2O production and emission. These results advance the knowledge on the microbial processes that regulate N2O emissions in inland waters and illustrate the integrated management of water quality and N2O emission.