Distinctive Patterns and Controls of Nitrous Oxide Concentrations and Fluxes from Urban Inland Waters.

Inland waters are significant sources of nitrous oxide (NO), a powerful greenhouse gas. However, considerable uncertainty exists in the estimates of NO efflux from global inland waters due to a lack of direct measurements in urban inland waters, which are generally characterized by high carbon and nitrogen concentrations and low carbon-to-nitrogen ratios. Herein, we present direct measurements of NO concentrations and fluxes in lakes and rivers of Beijing, China, during 2018-2020. NO concentrations and fluxes in the waters of Beijing exceeded previous estimates of global rivers due to the high carbon and nutrient concentrations and high aquatic productivity. In contrast, the NO emission factor (NO-N/DIN, median 0.0005) was lower than global medians and the NO yield (ΔNO/(ΔNO + ΔN), average 1.6%) was higher than those typically observed in rivers and streams. The positive relationship between NO emissions and denitrifying bacteria as well as the Michaelis-Menten relationship between NO emissions and NO-N concentrations suggested that bacteria control the net production of NO in waters of Beijing with N saturation, leading to a low NO emission factor. However, low carbon-to-nitrogen ratios are beneficial for NO accumulation during denitrification, resulting in high NO yields. This study demonstrates the significant NO emissions and their distinctive patterns and controls in urban inland waters and suggests that NO emission estimates based on nitrogen loads and simple emission factor values are not appropriate for urban inland water systems.