Research On Nitrogen And Phosphorus Pollution Budget In The Typical Xiaojiang Backwater Area,Three Gorges Reservoir Region

The Yangtze River becomes a reservoir with the accumulation of plant nutrients mainly composed of nitrogen and phosphorus in the backwater reservoir,causing eutrophication and algal bloom.Nitrogen and phosphorus is the main pollutants to cause of water eutrophication.Limiting the intake of exogenous and endogenous nitrogen and phosphorus pollutants isthe feasible way to control eutrophication in Three Gorges Reservoir Region.Therefore,it is of great significance to study the income and expenditure of nitrogen and phosphorus pollutants in typical bay of tributary in Three Gorges Reservoir Region to prevent and control eutrophication of tributary.This study takes the typical Kuwan Gaoyang Pinghu of Xiaojiang,the main tributary of the Three Gorges reservoir area,as the research object,and analyzes the distribution and morphology of nitrogen and phosphorus pollutants in the water,sediment and water fall zone of the typical reservoir bay of Xiaojiang.In this way,the nitrogen and phosphorus balance and distribution of the typical reservoir bay of the Xiaojiang tributary in the Three Gorges reservoir area are obtained.At the same time,the flux estimation and revenue estimation methods constructed in this paper are used to estimate the nitrogen and phosphorus fluxes in the Kuwanwan belt and the nitrogen and phosphorus balance in the Kuwan small watershed.The main findings are:（1）This paper proposes a new type of supporting structure-unloading plate type pile plate supporting structure.The supporting structure is mainly composed of a supporting pile,an unloading plate,a rib plate and a retaining plate.The new structure is based on the cantilever pile-support structure,and the unloading plate is added to reduce the earth pressure of the soil behind the pile and to increase the stable bending moment by the weight of the backfill.The reservoir water body is weakly alkaline throughout the sampling process.There is no significant difference in the spatial distribution of TN concentration at different points in Kuwan,but the difference in time distribution is significant,and the trend of each point is relatively consistent;The TN concentration at each point exceeded the water eutrophication threshold.The soil in the water-fluctuating zone of the study area showed neutral alkaline in the whole period of the dry-down period,and the soil organic matter content in the different points of the falling zone was significantly different.The nitrogen form of soil in the Xiaowan belt of the Kuwan Bay is mainly composed of NO3-N and the main form of phosphorus is Ca-P.The trends of soil Ca-P,IP and TP are basically the same.As the TP content increases,the IP and Ca-P content also increases,which is the main source of IP and TP in the study area.The sediment content of the reservoir in Kuwan Bay is relatively high,and it is easy to resuspend under external disturbance.The pH value of each point is in the alkaline range as a whole,and the organic matter content of sediments at different points is obviously different.（2）There was a significant positive correlation between soil organic matter content and TP and NO₃-N content in the water-fluctuating zone,indicating that organic matter and NO₃-N have similar input and output pathways.There was also a significant positive correlation between soil TP content and Fe/Al-P,Ca-P and OP contents in the water-slip zone,indicating that the increase of TP content in the soil of the water-fluctuating zone leads to the majority of phosphorus forms,especially the main component of inorganic phosphorus,Fe/Al.The increase in the content of-P and Ca-P also indicates that the main sources of increase in TP content are Fe/Al-P,Ca-P,and OP.（3）From March to September,the TN of most sediments in the Bay showed a state of income,which was represented by the“sink”of nitrogen in the overlying waters,while the TP of the sediments at most points showed an expenditure state,expressing as the"source"of the overlying water body phosphorus;The TP of all the water bodies in the reservoir bay is in a state of expenditure,especially in the central part of Kuwan（S2）,Kuwan Strait（S1）,Kuwan（S4）and Kuwan（S3）.The results of soil nitrogen and phosphorus fluxes in the fall-off period were as follows:total nitrogen flux was-4.27t,and total phosphorus flux was 0.96t.The soil nitrogen content in the fading zone during the dry period is generally increased and the quantity is large,which also indicates that the fading zone is an important nitrogen source in the reservoir water.The phosphorus content in the fading zone is generally reduced,and there is a possibility of loss.（4）In 2018,the total input of N and P in the small watershed of the bay was67.08 t/a and 44.91 t/a,among which the input of nitrogen and phosphorus brought by chemical fertilizer occupied the dominant position of nitrogen and phosphorus input in small watershed;the phosphorus input brought by atmospheric deposition The amount only accounts for 12.7%of the total phosphorus input in the small watershed,but the total nitrogen input is basically the same as the nitrogen input of the fertilizer.Therefore,in order to achieve a stable balance of total nitrogen and phosphorus in the basin,the focus is to strengthen the management of fertilizer input.