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

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 (N2O) variation and its emissions in Chinese river-reservoir systems. In this study, N2O spatiotemporal variations were investigated monthly in two reservoirs along the Wujiang River, Southwest China, and the emission fluxes of N2O were estimated. N2O production in the reservoirs tended to be dominated by nitrification, according to the correlation between N2O and other parameters. N2O 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 N2O in both reservoirs was over-saturated with respect to atmospheric equilibrium levels, suggesting that the reservoirs were net sources of N2O emissions to the atmosphere. The averaged N2O 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 N2O emission fluxes as compared to other lakes in the world. Downstream water of the dams had quite high levels of N2O saturation, and the estimated annual N2O emissions from hydropower generation were 3.60 × 105 and 2.15 × 105 mol N2O for the Wujiangdu and the Hongjiadu reservoir, respectively. These fluxes were similar to the total N2O emissions from the reservoir surfaces, suggesting that water released from reservoirs would be another important way for N2O 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 N2O spatiotemporal variations and emissions.