Nitrous oxide emissions from cascade hydropower reservoirs in the upper Mekong River

Abstract

Nitrous oxide is a powerful greenhouse gas, and its emissions from single reservoirs have been extensively studied; however, it still remains unclear about nitrous oxide emission patterns in cascade reservoirs. In this study, nitrous oxide emissions from cascade hydropower reservoirs were investigated using the thin boundary layer model in the heavily dammed upper Mekong River. Meanwhile, sediment denitrification for nitrous oxide production was analysed using the stable isotope method and the quantitative polymerase chain reaction method. Our results demonstrated that nitrous oxide emissions (0.47-1.08 μg m^-2h^-1) in the upper Mekong River were much lower than the global mean level (19.60 μg m^-2h^-1), but were increased by dam constructions; nitrous oxide emissions exhibited an increase trend along the flow direction in the cascade reservoirs. Sediment accumulation by dams supplied sufficient nitrogen substrates and organic carbon, creating hotspots of denitrification at the transition zone in reservoirs. As the elevation decreased, the increase in temperature enhanced microbial denitrification at the active zone, and thereby increased nitrous oxide production with the prolonged residence time. This study advanced our knowledge on nitrous oxide emissions from cascade hydropower systems.

Keywords: Cascade reservoirs; Denitrification; Elevation; Mekong river; Nitrous oxide.