三峡库区消落带N2O排放及其影响因素

为了探讨亚热带水库消落带N_2O的排放规律,选取三峡库区王家沟一典型消落带内3个高程(180、175和155 m)作为研究对象,采用静态暗箱和浮箱法进行了为期2 a的连续观测.175 m和155 m高程位于三峡库区消落带上,而180 m高程作为对照,为永不淹水的陆地.结果表明,各高程处的N_2O排放通量都表现出明显的季节变化,180 m高程处的春季N_2O排放最低;175 m高程在实验观测的第一年表现为单峰型的夏季N_2O排放高峰,次年在三峡水库实现最高蓄水位175 m后,表现为干湿交替和夏季N_2O高排放的双峰型;155 m高程处只呈现为夏季高N_2O排放的单峰态.另外,位于消落带上的175 m和155 m高程均表现为落干期N_2O排放大于淹水期.各高程处N_2O的年累积排放量为175 m(853.92 mg·m-2)180 m(336.69mg·m~(-2))155 m(324.69 mg·m~(-2)),与180 m高程对照相比,表明短期淹水会促进N_2O排放,而长期淹水则会抑制N_2O排放.相关性分析显示,陆地与消落带落干期的N_2O排放与各环境因子间无显著相关性,消落带淹水期排放与水温和风速呈极显著负相关.对影响陆地、消落带淹水期和落干期N_2O排放的因素进行主成分分析可知,消落带淹水期水体中可溶性氮素的分布是影响水面N_2O排放的最主要因素,而消落带落干期及陆地则是受土壤碳氮含量、土壤温度、湿度及pH等因素的共同影响和制约.

Features and Influencing Factors of N2O Emissions from Drawdown Area in the Three Gorges Reservoir

Three heights (180 m, 175 m and 155 m) located in a typical drawdown area in Wangjiagou of the Three Gorges Reservoir were selected for studying the N₂O emissions from subtropical reservoirs. The experimental period lasted two years from August 2010 to August 2012. The methods of static opaque chambers during the drainage period and floating chambers during flooding period were adopted in this study. The heights of 175 m and 155 m were both located in the drawdown area, whereas the 180 m height was located in the land as a control to 175 m and 155 m. N₂O fluxes showed clear seasonal trends at each height and remarkable differences were observed between the two years at the 180 m height. N₂O fluxes were lowest in spring at the 180 m height. N₂O fluxes showed a single-peak pattern with climax in summer in the first year, whereas a double-peak pattern with climax in summer and after dry-wet alternating in the next year after the highest water level of 175 m was succeeded in the Three Gorges Reservoir. N₂O fluxes presented a single-peak shape with summer climax at the 155 m height. Additionally, N₂O fluxes were higher during the drainage period than in the flooding period at both the 175 m and 155 m heights. The order of the annual N₂O cumulative emissions at the three heights was 175 m (853.92 mg·m^-2) > 180 m (336.69 mg·m^-2) > 155 m (324.69 mg·m^-2), and there was a notable difference between 175 m and 155 m, indicating that short-term flooding could increase N₂O emissions but long-term flooding could restrain N₂O emissions. Correlation analysis showed that there were no obvious relativities between N₂O fluxes and environmental factors in the land and during the drainage period. However, N₂O fluxes were significantly negatively correlated with water temperature and wind speed during the flooding period. Principal component analysis found that soil nutrient conditions and physicochemical properties were the most important factors for N₂O emissions in the land, the nitrogen distribution in water was a main determinant for N₂O emissions during the flooding period, and soil physicochemical properties and microbial activity importantly affected N₂O emissions during the drainage period in the drawdown area.