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

Inland waters are significant sources of nitrous oxide (N2O), a powerful greenhouse gas. However, considerable uncertainty exists in the estimates of N2O 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 N2O concentrations and fluxes in lakes and rivers of Beijing, China, during 2018–2020. N2O 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 N2O emission factor (N2O-N/DIN, median 0.0005) was lower than global medians and the N2O yield (ΔN2O/(ΔN2O + ΔN2), average 1.6%) was higher than those typically observed in rivers and streams. The positive relationship between N2O emissions and denitrifying bacteria as well as the Michaelis–Menten relationship between N2O emissions and NO3––N concentrations suggested that bacteria control the net production of N2O in waters of Beijing with N saturation, leading to a low N2O emission factor. However, low carbon-to-nitrogen ratios are beneficial for N2O accumulation during denitrification, resulting in high N2O yields. This study demonstrates the significant N2O emissions and their distinctive patterns and controls in urban inland waters and suggests that N2O emission estimates based on nitrogen loads and simple emission factor values are not appropriate for urban inland water systems.