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

Highlights: • Urban lakes in Beijing mainly act as a sink of N₂. • Urban inland rivers emit negligible N even under high nitrogen loading. • Low nitrogen-to-phosphorus ratios in the urban waters potentially favor N₂ fixation. • Low carbon-to-nitrogen ratios are not beneficial for N₂ production in urban inland waters. 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.