Characteristics and Influencing Factors of CH4 Emissions from the Drawdown Area of the Three Gorges Reservoir

Abstract

Five levels (180 m, 175 m, 165 m, 155 m, and 140 m) in a typical drawdown area in Wangjiagou in the Three Gorges Reservoir were selected to study CH4 emissions from subtropical reservoirs. The experimental period lasted two years from September 2010 to August 2012. The methods of static opaque chambers during the drainage period and floating chambers during flooding period were used in this study. The elevations of 175 m, 165 m, and 155 m were all located in the drawdown area, whereas the 180 m elevation was located in the land and never flooded. The 140 m elevation was permanently flooded and used as a control area. The results showed that the CH4 fluxes showed no significant trends at 175 m and 165 m in the first year of the experiment, while the fluxes showed a single peak pattern with the climax in the summer at 155 m and 140 m. At 175 m, the CH4 emissions showed a single peak pattern with the climax during its flooding period, and then showed not regular CH4 emission sources or sinks alternately in the second year, whereas the CH4 fluxes at 165 m, 155 m, and 140 m presented a single-peak shape with winter climax. During the entire observation period, the CH4 emission fluxes at 180 m were stable and showed no obvious peaks. In addition, CH4 fluxes were higher during the flooding period than in the drainage period at 175 m, 165 m, and 155 m.The order of the annual CH4 cumulative emissions at the five elevations was 140 m (99.58 kg·hm-2) > 155 m (82.98 kg·hm-2) > 165 m (65.38 kg·hm-2) > 180 m (6.32 kg·hm-2) > 175 m (4.27kg·hm-2), suggesting that the soil was more conducive to CH4 production when the flooding period was longer. Correlation analysis indicated that there were no significant correlations between CH4 fluxes and the soil carbon component and pH on land and during the drainage period but CH4 fluxes increased with the increase in soil water content. There was a significant linear negative correlation between CH4 emissions from the gas-water interface at 140 m and in water. The soil moisture content was one of the key factors affecting the CH4 fluxes during the drainage period, while during flooding period, the CH4 fluxes were regulated by flooding depth.