Coregulation of nitrous oxide emissions by nitrogen and temperature in China's third largest freshwater lake (Lake Taihu)

Abstract

Nitrous oxide (N2O) is a potent greenhouse gas and contributes to the loss of stratospheric ozone. However, the role of inland waterbodies in the dynamics of atmospheric N2O is poorly understood. We investigated N2O fluxes and their controlling factors in Lake Taihu, a large and shallow (2400 km2, 1.9 m depth) eutrophic lake in eastern China. Long-term measurements (2011–2016) revealed spatial and temporal variations in the lake surface N2O fluxes. The mean N2O flux from the lake was 3.5 ± 1.8 (mean ± SD) μmol m−2 d−1, with an annual N2O budget of 134.4 ± 69.8 Mg (106 g) yr−1. The highest N2O fluxes occurred in the eutrophic zone with significant anthropogenic N inputs, and the lowest fluxes occurred in the noneutrophic zone with no external N inflow. A seasonal pattern in N2O fluxes was observed only in the noneutrophic zone and was strongly correlated with water temperature. No seasonality in the N2O fluxes was observed in the eutrophic zone with high N concentrations in the water, indicating that N concentrations play a dominant role in regulating N2O fluxes compared to water temperature. The average N2O emission factor in Lake Taihu was 0.18%, with temporal and spatial variations negatively associated with N concentration but positively associated with the mass ratio of dissolved organic carbon to dissolved inorganic nitrogen. Our results suggest that anthropogenic activities strongly affect N2O fluxes in freshwater lakes.