Spatiotemporal variations of nitrous oxide (N2O) emissions from two reservoirs in SW China

Abstract

Abstract Greenhouse gas emissions from hydroelectric dams have recently given rise to controversies about whether hydropower still provides clean energy. China has a large number of dams used for energy supply and irrigation, but few studies have been carried out on aquatic nitrous oxide (N 2 O) variation and its emissions in Chinese river-reservoir systems. In this study, N 2 O spatiotemporal variations were investigated monthly in two reservoirs along the Wujiang River, Southwest China, and the emission fluxes of N 2 O were estimated. N 2 O production in the reservoirs tended to be dominated by nitrification, according to the correlation between N 2 O and other parameters. N 2 O saturation in the surface water of the Wujiangdu reservoir ranged from 214% to 662%, with an average fluctuation of 388%, while in the Hongjiadu reservoir, it ranged from 201% to 484%, with an average fluctuation of 312%. The dissolved N 2 O in both reservoirs was over-saturated with respect to atmospheric equilibrium levels, suggesting that the reservoirs were net sources of N 2 O emissions to the atmosphere. The averaged N 2 O emission flux in the Wujiangdu reservoir was 0.64 μmol m −2 h −1 , while it was 0.45 μmol m −2 h −1 in the Hongjiadu reservoir, indicating that these two reservoirs had moderate N 2 O emission fluxes as compared to other lakes in the world. Downstream water of the dams had quite high levels of N 2 O saturation, and the estimated annual N 2 O emissions from hydropower generation were 3.60 × 10 5 and 2.15 × 10 5 mol N 2 O for the Wujiangdu and the Hongjiadu reservoir, respectively. These fluxes were similar to the total N 2 O emissions from the reservoir surfaces, suggesting that water released from reservoirs would be another important way for N 2 O to diffuse into the atmosphere. It can be concluded that dam construction significantly changes the water environment, especially in terms of nutrient status and physicochemical conditions, which have obvious influences on the N 2 O spatiotemporal variations and emissions.