Characteristics And Mechanism Of Nitrogen Cycle In The Water Level Fluctuation Zone Of The Reservoir Under Anti-season Regulation

The water level fluctuation zone(WLFZ)of a reservoir has become a hot zone of the nitrogen biogeochemical cycle due to its special hydrological process,which has an important impact on the water environment and the water ecological health of the reservoir and downstream.The WLFZ is called the "liver" of the river ecosystem,and at the same time,it is considered to be nitrogen pollution due to the hydrolysis and mineralization of surface organisms.The duality of the WLFZ is particularly prominent under the anti-seasonal regulation of the reservoir.At present,the understanding of the mechanism of nitrogen migration and transformation in the fluctuating zone under antiseasonal regulation is relatively lacking,so that there is a lack of quantitative research on the process of nitrogen production and consumption.In this paper,the typical Three Gorges Reservoir bay WLFZ is used as the research object,through field monitoring,experiment,numerical simulation and other methods,carried out the research on the characteristics of nitrogen cycle and its mechanism and process simulation of the WLFZ under anti-seasonal regulation.The main research results are as follows:(1)The characteristics of soil and water environment change in the WLFZ with antiseasonal regulation were explored.The rainfall in the study area is concentrated in the low water level period(May to July)in the WLFZ.During this period,the reservoir water contains a large amount of sand,and the total nitrogen content is the annual peak,which reduces the dissolved oxygen in the water and makes the suspended sediment of the water have great nitrification potential.Chlorophyll a in the water is the highest in the mid-term of recession period(from March to April),nitrate is consumed in a large amount,and dissolved oxygen is restored.Accordingly,the content of organic nitrogen is higher in March to May.Dissolved oxygen and temperature at the water-soil interface show a reverse trend with the change of water level,which are the key factors affecting the nitrogen transformation in soil.Nitrogen flux is positively correlated with water level,and water infiltrates into the soil during high water level,and vice versa.The soil nitrogen functional microbial abundance is mainly affected by temperature and p H,and it is generally higher in the low water stage.In the soil,ammonia nitrogen is dominant in the low water level period and the flooded period,while nitrate nitrogen is dominant in high water level period and the recession period.(2)The mechanism of nitrogen cycle in the WLFZ under anti-seasonal regulation are clarified.During the low water level period,the denitrification of soil is weak,while the denitrification potential of suspended sediment in water is large,which resulted in the increase and decrease of inorganic nitrogen in soil and water.During the flooding period,surface soil mineralization is dominant,ammonia nitrogen in soil increases significantly,and total inorganic nitrogen in water increases.During high water level period,the water transparency is high,and the nitrogen forms are basically stable,while the denitrification rate of the soil is high and the nitrate nitrogen is significantly reduced in the anoxic environment.In the early stage of water dewatering,the nitrate nitrogen in water drops sharply due to the absorption of phytoplankton,while the soil increases to a certain extent due to denitrification.In the later stage,the water is turbid and the oxygen capacity is low,and the denitrification of soil and water is strong.However,due to the influence of rain in early May,the total inorganic nitrogen in soil and water increased to a certain extent.In a complete hydrological year,the potential pollution caused by the anti-seasonal regulation of the reservoir does not cause the accumulation of nitrogen in the WLFZ and the increase of the nitrogen load in the water body,because the soil in the flooded period and the suspended sediment in the low-water period have strong nitrogen removal ability.(3)A model of the process of nitrogen generation and depletion in the WLFZ under the anti-seasonal regulation was established.And the main parameters of the model were calibrated and verified through actual measurement of long-series water temperature and nitrogen indicators.The nitrogen generation and consumption process in the WLFZ of the Three Gorges Reservoir and its response to the key influencing factors under an annual hydrological operation were simulated and analyzed,and the adaptability of the model in the simulation of anti-season regulation of reservoir nitrogen cycle was verified.