[Distribution and Emission of Nitrous Oxide (N₂O) in Three Gorges Reservoir and Downstream River]

Nitrous oxide (N₂O) is an important greenhouse gas and has a great impact on global warming. Affected by human activities, rivers, and reservoirs have become active sites for N₂O production and emission. In Three Gorges Reservoir and the downstream Yangtze River, the concentrations and fluxes of dissolved N₂O were investigated in September-October 2009 and October 2016 to identify the factors controlling the distribution of N₂O in the reservoir and the effect of damming and reservoir operation on N₂O emission from the Yangtze River. The dissolved N₂O concentration of the reservoir ranged from 9.74 to 16.36 nmol·L^-1 with an average of (12.49±1.75) nmol·L^-1 in the surface water (about 0 m) and ranged from 9.99 to 14.00 nmol·L^-1 with an average of (11.21±0.91) nmol·L^-1 in the bottom water (ranging from 6 m to 103 m). The dissolved N₂O in the water column of Three Gorges Reservoir varied little, and no significant difference was noted between the overall N₂O concentration in the reservoir and that in the downstream river. A positive correlation was noted between N₂O and nitrate (NO₃^-) (P<0.01), although negative correlations were found between N₂O and ammonium (NH₄⁺) (P<0.05) and nitrite (NO₂^-) (P<0.01) in Three Gorges Reservoir. The surface N₂O in the Three Gorges Reservoir was overall saturated, with saturation ranging from 122% to 170% in 2009 and from 114% to 187% in 2016. The mean emission rate of N₂O from the surface water was (4.6±2.4) μmol·(m²·d)^-1 in 2016 and (16.6±4.9) μmol·(m²·d)^-1 in October 2009. The Three Gorges Reservoir is a non-negligible source of atmospheric N₂O. No obvious difference was noted between upstream and downstream N₂O concentrations, which implies that no degassing emission occurs when water passes through turbines and spillways.