Low diffusive nitrogen loss of urban inland waters with high nitrogen loading

Abstract Little is known about the exchange of gaseous nitrogen (N2) with the atmosphere from urban inland waters, which are characterized by low carbon-to‑nitrogen ratios and low nitrogen-to‑phosphorus ratios. Here, we studied diffusive nitrogen loss based on the measurement of dissolved N2 concentrations and related gene abundance of N2 production and fixation in rivers and lakes in the megacity of Beijing, China, between 2018 and 2020. The excess dissolved N2 (△N2) ranged from −51.2 to 56.8 μmol L−1 (average − 0.03 ± 13.8 μmol L−1), and approximately 43% of the river samples and 72% of the lake samples being undersaturated with N2, suggesting that the lakes mainly acted as a role of N2 sink. The N2 removal fraction (△N2/DIN, average 3.5 ± 4.3%) at the sites of rivers with positive △N2 was lower than that in other rivers around the world. The average N2 flux (0.8 ± 23.9 mmol m−2 d−1) in the urban rivers was also lower than that in other rivers. The low carbon-to‑nitrogen ratios in Beijing inland waters are not beneficial for N2 production during denitrification, and low nitrogen-to‑phosphorus ratios potentially favor N2 fixation with a high abundance of the nitrogenase nifH gene in the sediment, resulting in low net N2 production. The traditional paradigm is that rivers constantly lose vast N to the atmosphere via denitrification and anammox, but this study indicates that urban inland rivers emit negligible N even under high nitrogen loading.