Wuliangsuhai Lake is one of the representative inland freshwater lakes in grassland area in China.Recently,the lake is shrinking and eutrophication has become serious.This paper interpreted the spatial distribution patterns of total nitrogen（TN）,ammonium nitrogen(NH+4-N),nitrate nitrogen(NO-3-N) and organic nitrogen（Org-N） in surface water and sediment cores systematically.The results revealed that the average content of TN was 1.78mg/L and the main inorganic nitrogen pollutant was NH+4-N which accounted for more than 69% of the inorganic nitrogen in surface water;spatial distribution pattern of nitrogen in surface sediment indicated that the nitrogen content in the west of the lake was higher than in the east and the main species of nitrogen was Org-N which was more than 95% of TN.Furthermore based on the Sediment Quality Estimation Guide which was enacted in terms of ecological toxic effects by Department of Environment and Energy of Ontario Province,Canada,1992,the content of TN in surface sediment has certain ecological toxic effects which are more serious;the content of nitrogen decreased with depth and the eutrophication process existed in three periods which revealed that the acceleration of the eutrophication process dated back to the 1990s.The content,speciation distribution and spatial distribution pattern of TN in Wuliangsuhai Lake′s surface water and sediment cores indicated that its pollution was characterized by exogenous and endogenous pollution.The exogenous pollution was mainly the drainage water from farmland and industrial wastewater,which accounted for 50% and 35% of the TN load respectively,pouring into the lake through the main drainage canals,the eighth and the ninth drainage canal.And under certain conditions,the endogenous load could release nitrogen into surface water and make it the main pollution source.Besides the exogenous and the endogenous pollution,the intense evaporation,which accelerated the eutrophication process of Wuliangsuhai Lake,was also a key factor which caused nitrogen accumulation in the surface and modern sediments.
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