[Distribution and Emission of Nitrous Oxide (NO) in Three Gorges Reservoir and Downstream River].

Nitrous oxide (NO) 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 NO production and emission. In Three Gorges Reservoir and the downstream Yangtze River, the concentrations and fluxes of dissolved NO were investigated in September-October 2009 and October 2016 to identify the factors controlling the distribution of NO in the reservoir and the effect of damming and reservoir operation on NO emission from the Yangtze River. The dissolved NO concentration of the reservoir ranged from 9.74 to 16.36 nmol·L with an average of (12.49±1.75) nmol·L in the surface water (about 0 m) and ranged from 9.99 to 14.00 nmol·L with an average of (11.21±0.91) nmol·L in the bottom water (ranging from 6 m to 103 m). The dissolved NO in the water column of Three Gorges Reservoir varied little, and no significant difference was noted between the overall NO concentration in the reservoir and that in the downstream river. A positive correlation was noted between NO and nitrate (NO) (<0.01), although negative correlations were found between NO and ammonium (NH) (<0.05) and nitrite (NO) (<0.01) in Three Gorges Reservoir. The surface NO 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 NO from the surface water was (4.6±2.4) μmol·(m·d) in 2016 and (16.6±4.9) μmol·(m·d) in October 2009. The Three Gorges Reservoir is a non-negligible source of atmospheric NO. No obvious difference was noted between upstream and downstream NO concentrations, which implies that no degassing emission occurs when water passes through turbines and spillways.