Studies On The Release Characteristics And Influencing Factors Of Nitrogen And Phosphorus For Typical Plants Of Three Gorges Reservoir Area After Flooding

The Three Gorges Reservoir adopts the operation and dispatching mechanism of"storing clear water and discharging muddy water".The reservoir began to store water at the end of September,and drained water in January（water level 175 m）the following year.The water level elevation gradually declined.In June,the water level elevation fell back to the lowest 145 m,thus forming a vertical drop zone of 30 m with an area of 349 km2 on both banks.As the ecotone of water and land ecosystems,the ebb and flow zone has an important impact on the ecological environment of the reservoir area.Because the fluctuation zone is the convergence zone of runoff in the reservoir area,nutrient elements and some pollutants are gathered here.At the same time,vegetation and soil in the ebb-flow zone can absorb nutrients and ultimately affect the water environment quality of the reservoir area.During the outcrop period of the Three Gorges Reservoir area,vegetation restoration and vigorous growth,interception of nitrogen and phosphorus nutrients entering the water body,thus reducing the water environmental pollution in the reservoir area;during the flooding period,the vegetation in the ebb-flow zone declined,resulting in the decomposition and release of nitrogen and phosphorus and other elements after some pollutants entered the water body,and there was also a complex process of material transfer and exchange between the overlying water and soil in the ebb-flow zone.Therefore,the study of nitrogen and phosphorus release characteristics and influencing factors after vegetation flooding in the ebb-flow zone provides a scientific basis for evaluating the environmental effects of the ebb-flow zone in the reservoir area,and is of great significance for formulating an appropriate management system of the ebb-flow zone.In this study,the characteristics of nitrogen and phosphorus release and the influencing factors of typical plants in the fluctuation zone of reservoir area after flooding were discussed through indoor simulated flooding experiments.The main conclusions are as follows:（1）A 30-day indoor simulated immersion experiment of typical plants in the ebb and flow zone of the Three Gorges Reservoir was conducted.The results showed that the color of the water body was deepened,the odor was produced and the dissolved oxygen in the water was reduced by the death and decay of the plants.The concentration of total phosphorus,soluble total phosphorus,total nitrogen,soluble total nitrogen,ammonia nitrogen and nitrate nitrogen in the overlying water changed greatly in the early stage of flooding,and tended to be stable in the later stage;the release of nitrogen and phosphorus reached the peak value after soaking for one day.Dissolved nitrogen is the main nitrogen released to the overlying water during the process of plant flooding and decomposition,accounting for more than 50%of total nitrogen;particulate phosphorus is the main phosphorus released to the overlying water during the process of plant flooding and decomposition,accounting for more than 85%of total phosphorus.（2）On the basis of indoor simulated immersion test,the environmental conditions of immersed plants were controlled,and the factors affecting the release of carbon,nitrogen and phosphorus from plant flooding were discussed.The results showed that under neutral conditions,the concentration of total nitrogen in overburden water was higher,and under acidic conditions,the concentration of total phosphorus in overburden water was the highest,and acidic environment contributed to the release of phosphorus;at 15-25 C,the higher the temperature was,the more nitrogen and phosphorus released after plant flooding;after three plants flooded,the change of total nitrogen in overburden water was similar.The relationship between total nitrogen concentration and plant species was different between high concentration and low concentration groups.In the process of root flooding,when the concentration of overlying water and salt was higher,total nitrogen released more.In the process of Bidens bipinnata flooding,the peak value of total nitrogen concentration in overlying water was:high concentration>low concentration.During the flooding process of Xanthium sibiricum,the concentration of total nitrogen in overlying water of low concentration experimental group was higher than that of high concentration experimental group.When Bermuda grass and Bidens pilosa were taken as research objects,it was found that the concentration of total phosphorus in overlying water of high concentration experimental group was higher than that of low concentration experimental group,while Xanthium sibiricum was taken as research object,the concentration of total phosphorus in overlying water of low concentration experimental group was higher than that of high concentration experimental group;the concentration of total nitrogen in overlying water of experimental group decreased when the experimental group changed from transparent soaking to shading soaking soaking.Small.Under shading treatment,the concentration of overlying water gradually decreased after reaching its peak value,and the total nitrogen concentration of overlying water during the whole flooding process was lower than that of the experimental group under light treatment,which indicated that light could promote the release of nitrogen from plants to the surrounding environment.（3）The total nitrogen and total phosphorus contents of soil and plant before and after immersion under different treatment conditions were measured,and the cumulative release of nitrogen and phosphorus from soil and plant during immersion was calculated.The results showed that the total phosphorus content increased and the total nitrogen content decreased in the process of submergence of Bermudagrass,Bidens pilosa and Xanthium sibiricum.During the process of submergence,the soil would release nitrogen to the surrounding environment and phosphorus would be absorbed to the soil.（4）High-throughput sequencing of overlying water under different treatment conditions was carried out to obtain the changes of microbial community structure during flooding.The results showed that the species and relative abundance of microorganisms in overlying water of three plant experimental groups were compared comprehensively during flooding.It was found that the relative abundance of Acinetobacter and Copperacillus was relatively high,and they were closely related to denitrification process.The community structure of microorganisms in overlying water is mainly affected by NO₃^-concentration.