Methane and nitrous oxide emissions from the drawdown areas of the Three Gorges Reservoir

To investigate the effect of dam impounding on CH₄ and N₂O emissions from the drawdown area of the Three Gorges Reservoir (TGR), CH₄ and N₂O fluxes were measured at the elevations of 180 m, 175 m, 165 m and 155 m (above the sea level) from September 2010 to August 2012 using the static chamber technique. The elevations of 175 m, 165 m and 155 m were located in the drawdown area and the non-flooded elevation of 180 m was taken as the control for the drawdown area. The drawdown area studied here acted as the sources of CH₄ and N₂O as a whole. Seasonal, inter-annual and spatial CH₄, but not N₂O, fluxes varied significantly between September 2010 and August 2012. The CH₄ fluxes were highest in winter but lowest in summer, and significantly higher in the wet year than in the dry year. The annually cumulative CH₄ emissions were 14.09 ± 4.27, 12.61 ± 3.59, 68.92 ± 13.09 and 77.41 ± 9.42 kg CH₄ ha^-1 from the elevations of 180 m, 175 m, 165 m and 155 m, respectively. Compared with 180 m elevation, the annual CH₄ emission was insignificantly decreased by 11% at 175 m elevation (P > 0.05), and substantially increased by 389% and 449% at 165 m (P < 0.05) and 155 m elevations (P < 0.05), respectively. The annually cumulative N₂O emissions were 4.74 ± 1.78, 7.43 ± 2.57, 3.39 ± 1.05 and 8.83 ± 1.95 kg N₂O ha^-1 from the above corresponding elevations, respectively, and there were no significant differences among the four elevations (P > 0.05). These results showed that CH₄ emissions were increased but N₂O emissions were not affected in the drawdown area after the dam impounding in the TGR, and CH₄ emissions were increased with the decrease in the elevations while N₂O emissions were not affected by the elevation in the drawdown area.