Abstract: Nitrogen (N) dynamics at different interfaces in canyon reservoirs during the thermal stratification period substantially influence the authentic quantification of N₂O emission in the cascade reservoir system. We examined various N species (NO₃^--N, NH₄⁺-N), dissolved N₂O concentration, and dissolved oxygen (DO) in water samples collected from three reservoirs (Hongjiadu, Dongfeng, and Wujiangdu) during thermal stratification period in the upper reaches of Wujiang River. The results showed that N₂O concentrations were 14.3-64.4, 16.5-35.7, and 17.0-70.8 nmol·L^-1 in Hongjiadu, Dongfeng and Wujiangdu reservoirs, respectively. Theseresults indicated that all three reservoirs act as N₂O source to the atmosphere. There was a significant correlation between ΔN2O and apparent oxygen utilization (AOU) in the whole profile of Dongfeng reservoir as well as above the hypoliminion (0-58 m) of Wujiangdu reservoir. This suggested that N₂O production is primarily dominated by nitrification. The vertical distribution of DO, NO₃^--N, and N₂O in hypoliminion of Wujiangdu and Hongjiadu reservoirs showed that N₂O production in the hypoliminion of Wujiangdu Reservoir could mostly result from denitrification driven by insitu NO₃^-, while that of Hongjiadu Reservoir could mainly result from denitrification driven by the exogenous NO₃^- from upper layer water. N₂O emission fluxes in the waterair interfaces in Hongjiadu, Dongfeng and Wujiangdu reservoirs were 0.4, 0.5 and 0.4 μmol·m^-2·h^-1, respectively. They were significantly higher than former measured values that conducted ten years ago. This implies that the enhanced N2O emission could presumably come from reservoir aging, water regulation of the reservoirs, and enhanced biogeochemical transformation of organic matter under climate changes. The N₂O emission fluxes during power generation in July were 0.19×10⁴, 1.6×10⁴ and 6.7×10⁴ mol in the outflow waters of Hongjiadu, Dongfeng and Wujiangdu reservoirs, respectively. The enhanced discharge of N₂O from outflow waters may be affected by the joint scheduling and water regulation between the reservoirs in the cascade-developed river-reservoir system.

Key words: cascade reservoir, greenhouse gas, nitrogen cycling, hypoliminion, nitrous oxide.