DISTRIBUTION AND FORMATION OF SOIL NITROGEN IN WATER LEVEL FLUCTUATION ZONE OF DEEP-LARGE RESERVOIR IN LANCANG-MEKONG BASIN OF LONGITUDINAL RANGE-GORGE REGION SOUTHWEST CHINA

The quantitative study of nitrogen forms is an essential prerequisite for soil nitrogen biogeochemical function and reservoir ecological risk analysis. Taking the Xiaowan Reservoir of longitudinal range-gorge region as example, the stepwise extraction and separation were used to investigate the content of different forms of nitrogen in the soil. We analyzed their distribution characteristics, influencing factors, and changes before and after submergence in the WLFZ. The content of TN in the soil increased with decreasing altitude, and it was higher during the dry period than that measured during the flooding period. The contents of the observed four forms of nitrogen were higher during the dry period than the flooding period and content could be ranked as follows: strong oxidant exchanged form of nitrogen (SOEF-N) > weak acid exchanged form of nitrogen (WAEF-N) > strong alkali exchanged form of nitrogen (SAEF-N) > ion exchanged form of nitrogen (IEF-N). Excluding WAEF-N and SAEF-N, the N form contents increased with decreasing altitude. Correlation analysis showed that the temperature, DO and pH of the water body were the main factors affecting soil physical and chemical factors (SOM, TP and temperature) in the WLFZ. Different forms of nitrogen in the WLFZ at different altitudes have obvious concentration gradients that are seasonally different, and are affected by submergence and soil characteristics. Therefore, it is suggested that attention should be given to the monitoring of biogenic elements in the WLFZ during the operation and management of large high dam reservoirs in the longitudinal range-gorge region.