Study On The Change Mechanism Of Nitrogen Forms In The Sediments Of Wuliangsu Sea During The Freezing Period

Nitrogen play an important role in the management of eutrophication lakes.The nitroge coversion impacts bioavailability and the exchange of sediment-overlying water.Wuliangsu lake is coverd over 90%by plants,belonging to a typical grass-type eutrophicated lake.Dead plants debris further increase the sediment nutrient stock in winter.However,under-ice nitrogen conversion is lacking.To understand the nitrogen variations during the ice-on season and its potential impactions in Wuliangsuhai,we performed the nitrogen Spatial-temporal variation by analyzing ion-exchangeable form(IEF-N),Weak acid extractable form(WAEF-N),Strong alkaline extractable form(SAEF-N),Strong oxidant extractable form(SOEF-N)and residue nitrogen(Res-N),based on samples collected before,in the middle,and at the end of ice-on season separately.Hierarchical Cluster Analysis and Relevance Analysis were used to help elucidate the main causes and implications ofunder-ice N variation.The results clearly show as follow:(1)Res-N accounted for the majority of the sedimentary nitrogen(54%～93%),and it more than 86%in the later period.SOEF-N accounted for the majority of the TAN(49-81%).While the sum of IEF-N,WAEF-N and SAEF-N only accounted for less than 15%of the TN during ice-on season.(2)TN content decresed From northwest to southwest-TN contents in four regions showed the order of Ⅳ>Ⅱ>Ⅰ>Ⅲ.IEF-N and WAEF-N contents of in IV is significantly greater than that others.SAEF-N and SOEF-N high spatial disparities across the lake.Res-N is similar to TN in spatial distribution.(3)Sediment N showed an obvious increase in the total ice-on season.TN increased 2.7-3.7 times from Ⅰ to Ⅲ regions.Res-N contributed to the majority of the accumulated sediment N.Res-N increment accounted for 96-124%of TN inⅠ～Ⅳ regions.Res-N increase 1.3-6.3 times than before the ice-on.TAN variation showed obvious difference in different ice-on stage(4)The TAN conversion and the exchange between sediment overlying water caused TAN variation.IEF-N and WAEF-N are most easily exchange between sediment and overlying water.Res-N increase was mainly induced by the debris of macrophytes.(5)Res-N accumulation in sediments,and the subsequent migration and transformations between sediment in ice-on season,supports nitrogen for plants.Control the internal nitrogen more difficult.Sediment N accumulated induced by the debris of macrophytes,which plays an important role in internal recycle.The research is valuable for controlling the internal nitrogen in Wuliangsuhai and other shallow seasonally-frozen lakes that have long suffered from macrophyte phytoplankton.