Study On The Variation Law Of Nitrogen Content In The Sediment Of Wuliangsuhai During The Plant Growth Period

It is of great significance to study the variation of sediment nitrogen content for eutrophication control.Wuliangsu Lake is a typical phytoplankton-dominated eutrophication lake.Plant growth and wilting have an important influence on the changes of nitrogen content in sediment.However,there are few studies on the change of nitrogen content and morphology in sediment during plant growth period.It is difficult to determine the variation of nitrogen fractions in sediment during plant growth period in Wuliangsu Lake,and it is difficult to provide scientific basis for the treatment of Wuliangsu Lake.In this paper,the Wuliangsu Lake was divided into five regions,i.e.,Region I(not vegetated without water flow),Region II,III,V(densely vegetated with external water flow)and Region IV(densely vegetated without water flow).The vertical stratified sampling of the sediment in the three periods of spring,late summer and late autumn was carried out.Then,the contents of ion-exchangeable form(IEF-N),weak acid extractable form(WAEF-N),strong alkaline extractable form(SAEF-N),strong oxidant extractable form(SOEF-N),residue nitrogen(Res-N)and total nitrogen(TN)were determined by continuous extraction method to determine the temporal and spatial variation of different nitrogen fractions during plant growth period of Wuliangsu Lake.Based on the methods of comparative analysis,correlation analysis and diffusion theory analysis,the main driving factors of sediment nutrient change were further analyzed.Research showed that:(1)Res-N was the main nitrogen fraction in sediments of Wuliangsu Lake during plant growth period,accounting for 69-73%of TN.SOEF-N was the main component of total available nitrogen(TAN),accounting for 55-57%of TAN.The order of TAN contents in different fractions was SOEF-N>IEF-N>SAEF-N>WAEF-N.(2)The sediment nitrogen accumulation was serious in spring,late summer and late autumn.Nitrogen accumulation in sediment caused by early exogenous pollution has not changed.In the horizontal direction,the nitrogen content showed a decreasing trend along the flow direction.The nitrogen content of sediment in Region Ⅲ and Region Ⅴ ware higher than that in other regions,while the nitrogen content of sediment in Region Ⅱ was lower.Region Ⅰ was less affected by exogenous factors,nitrogen content in the sediment was low in this region;Plants in Region Ⅲ ware lush,and the regional environment was relatively closed and independent.The nitrogen content in sediment is high.Except for the total drainage outlet,the nitrogen content decreased obviously with increasing depth in the vertical direction.(3)The TN content of sediment during plant growth period showed an accumulation trend,and the TN content of sediment in other regions increased except the Region Ⅴ.In April-September,TN in the sediment of Region Ⅳshowed a decreasing trend,while that of Region Ⅰ showed an increasing trend,but the variation was small.TN in the sediment of Region Ⅱ,Ⅲ and Ⅴ showed an increasing trend.The change trend in September-November was opposite to that in April-September.The TN content in the sediment of Region IV increased,and the TN content in the sediment of Region Ⅰ decreased.The TN content in Region Ⅱ,Ⅲ and Ⅴ decreased significantly.(4)The comparative analysis of regional comparison,correlation analysis,diffusion theory and the change of total plant-sediment nitrogen showed that plants were the main driving factor for the change of sediment nitrogen in the densely vegetated without water flow region.Plant absorption in April-September resulted in the decrease of nitrogen content in sediment,while plant decomposition in September-November resulted in the increase of nitrogen content in sediment.The influencing factors of sediment nitrogen change in water circulation region are more complex,which are affected by external pollution and hydrodynamic force in addition to plants.