Dramatic source-sink transition of N 2 O in the water level fluctuation zone of the Three Gorges Reservoir during flooding-drying processes

Abstract

Biogeochemical cycling of nitrous oxide (N₂O), a significant greenhouse gas (GHG), can influence global climate change. The production and emission of N₂O mediated by hydrological regimes is particularly active in water level fluctuation zones (WLFZs). However, the hydrological mechanisms affecting N₂O transformation and production across the water-sediment micro-interface remain unclear. In this study, intact sediment cores from the WLFZs of the Three Gorges Reservoir (TGR) were incubated for 24 days in a laboratory microcosm to identify the effects of the flooding-drying processes on the yield and emission of N₂O. Results showed a source-sink transition of N₂O in the first 1.5 days during the flooding period, with the water column subsequently acting as a sink relative to the atmosphere in the following experimental period. The source-sink transition was ascribed to changes in oxygen concentration in the water column and sediment regulation of NO₃⁻-N transformation, resulting in denitrification and N₂O production. Preliminary estimates on the mass budget of N₂O in a typical WLFZs of the TGR showed slight emission fluxes, ranging from 13.08 to 43.08 μmol m⁻² from flooding period to drying process. Although these N₂O emissions were relatively low, the emission peak detected during the initial period (first 1.5 days) of the flooding phase provides important knowledge on the mitigation of GHG emissions from hydropower sources, which should be incorporated into future reservoir operations.