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 (f_bD) 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 f_bD 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.