Study On The Deposition Characteristics And Sources Of Organic Nitrogen In The Sediments Of Dianchi Lake

With the continuous development of society and economy,dramatic human activities have led to a rapid increase in amount of nurtrients into lake that makes the eutrophication of lake more serious.Eutrophication has become a hot ecological and environment problem.Dianchi Lake is the largest lake in Yunnan Province with serious lake eutrophication.The input of excessive nutrients has caused the serious eutrophication in Dianchi Lake.Sediments play a key role in nitrogen reservoirs in lakes while the main form of nitrogen in lake sediments is organic nitrogen,it accounts for about 93%of total nitrogen in sediments.In this study,we selected organic nitrogen in sediments in Dianchi Lake as research objects,in July 2014,three sedimentary column were collected and water samples in the southern,eastern and northern lakes of Dianchi Lake.The distribution of organic nitrogen(ON)in sediments and its sources were studied.We carried out simulated nitrogen mineralization in sediments to investigate the effects of temperature and algal on the process of endogeneous nitrogen release.Meawhile,we discuss the influencing factors affecting exogenous input of ON in sediments by STRIPAT(stochastic impacts by regression on population,affluence and technology)Model.The main findings are as follows:(1)The content of organic nitrogen(ON)in the sediments of Dianchi Lake is ranging from 297.8 to 6140.3 mg kg-1.In the southern,northern and eastern lakes,The content of the organic nitrogen(ON)showed a growth trend from bottom to surface sediments With the decrease of sediment depth,ON showed a rapid growth trend but the age of the beginning of growth was different,which was characterized by the trend of the north earlier than the east and the south.The vertical distribution of nitrogen(ON)content is also uneven,showing the trend of the south larger than the north than the east.(2)The analysis of the correlation between organic nitrogen(ON)and the molecular proxy index of n-alkanes.The results showed that in the southern part of Dianchi Lake,ON in sediment are mainly from endogenous bacteria,algae,and submerged and floating plants with fewer external inputs.After 1980,the source of ON was mainly from a large amount of nitrogen fertilizer.The ON in sediments in eastern region were mainly derived from its endophytic algae and terrestrial higher plants dominated by woody plant.The sources of ON in the northern lake area of Dianchi Lake are mainly endogenous phytoplankton and algae.(3)Based on the sediment mineralization simulation experiment,we dicussed the influence of temperature and algae on nitrogen mineralization in sediments.The results showed that NH4+-N was the main form of nitrogen release from the sediment,NH4+-N and pH have a negative correlation in the overlying water.The pH value in the overlying water of experimental group with added algae decreased more than that group without added algae.Therefore,the pH in the overlying water in the northern part of Dianchi Lake is slightly higher than that in the southern,as there is a higher eutrophication and larger number of algae in the northern part of Dianchi Lake.At the same time,adding algae into sediments also promoted the increase of nitrogen mineralization in sediments.Therefore,there is a phenomenon that the total nitrogen content in the overlying water in the north of Dianchi Lake was higher than that in the south,but the nitrogen content of the sediment in the north of Dianchi Lake was lower than that in the south.One of the reason is that the northern part of Dianchi Lake suffered a serious eutrophication problem with a large number of alage in the overlying water,the more decay of algae accumulates in sediments will promote the process of sediment nitrogen mineralization.Meanwhile,the experiment also showed that the higher temperature producted the more amount of nitrogen mineralization.Therefore,with the trend of continue global warming,the temperature increase will casue more release of endogenous nitrogen in the sediments of lake.(4)Using the STRIP AT model analyzed the influencing factors that affected the organic nitrogen sedimentary largest in different region of Dianchi Lake.The results showed that in the southern lake area,the most significant impact for organic nitrogen sedimentary were nitrogen fertilizer use and rainfall in this area.In the east of Dianchi Lake,nitrogen fertilizer use has a significant impact on organic nitrogen sedimentary,but rainfall has a slight negative effect on the organic nitrogen sedimentary of the eastern lake.In the northern lake area,temperature and rainfall have the great influence on organic nitrogen sedimentary of the northern lake.