Study On The Source And Transformation Process Of Nitrogen In Water Of Caohai,Guizhou

Wetland is one of the three major ecosystems of the earth,which has unique environmental regulation function and benefits,and is also an important place of global nitrogen cycle.As an important biogenic element of wetland,nitrogen content directly affects the primary productivity of wetland system.Caohai in Weining,Guizhou Province is the largest karst wetland in China,which plays an important role in the local natural and social environment.However,due to the interference of human activities,the wetland has a serious trend of swamping,and the water quality has gradually deteriorated.Total nitrogen is one of the main factors affecting the water quality of Caohai,and nitrogen pollution has become the primary pollution problem faced by Caohai.Therefore,this paper selected Caohai in Weining County of Guizhou Province as the main research object,collected Caohai surface wetland water by grid point method,and combined with the monitoring results of surrounding surface water,wetland surface sediments and aquatic plant samples,to explored the source of nitrogen in water and its migration and transformation process,the research results can provide reference for the water environment protection of Caohai.The main conclusions are as follows:(1)The results of determination and analysis of total nitrogen and various forms of nitrogen in wetland water show that the concentration of total nitrogen at most points in the lake area can not meet the environmental quality standard of class I surface water,and the content seriously exceeds the standard.In wet season,inorganic nitrogen(NH4+-N、NO3--N)was the main form of nitrogen in wetland water,and the two forms of inorganic nitrogen had higher concentration distribution in the west of wetland;In dry season,dissolved organic nitrogen(DON)is the main form of nitrogen in wetland water,and the high value area of DON appears in the south area.Combined with the determination of total nitrogen concentration in the river into the lake,the results show that the wetland water in wet season is mainly affected by exogenous input,while in dry season it is mainly affected by the release and transformation of wetland endogenous.(2)The results of Hydrochemistry and stable isotope analysis show that the nitrate pollution of Caohai water body comes from the joint influence of various human sources.In wet season,δ15N-NO3-and δ18O-NO3-values of some water samples were mainly distributed in soil organic nitrogen,feces and sewage,as well as the mixed endmember area of the two sources.In dry season,δ15N-NO3-andδ18O-NO3-values of most water samples were outside the endmember area of nitrate pollution sources,and only a few samples were distributed in soil organic nitrogen and atmospheric precipitation endmember area.Combined with the determination of nitrogen and oxygen isotope values in the rivers flowing out and into the lake,the results show that the input of feces and sewage has a greater impact on wetland water in wet season,while the input of external sources has less impact in dry season.(3)The results of the determination of nitrogen content and nitrogen isotopes in aquatic plants show that the water nitrogen pollution in wetland is heavy in the high water period,which causes the increase of nitrogen absorption by plants,In the process of nitrogen utilization by plants,isotope fractionation effect occurs.In dry season,a large number of aquatic plants decline,and the settlement of plant residues becomes one of the main nitrogen pollution sources in sediments and overlying water;The results of nitrogen content and nitrogen isotope in sediments show that,the concentration of total nitrogen in sediments was high,which mainly came from the deposition of aquatic plant residues,the input of soil organic nitrogen and industrial synthetic fertilizer.The adsorption and release of nitrogen in sediments also affected the concentration of nitrogen in overlying water.(4)The relative contribution rates of different nitrogen sources in wetland water were estimated by using SIAR model.The results showed that the contribution rates of feces and sewage,atmospheric precipitation,soil organic nitrogen and agricultural fertilizer in wet season were 61.34%,27.24%,6.44% and 4.98% respectively.The contribution rates in dry season were 34.07%,39.89%,14.57% and 11.47%respectively.Feces,sewage and atmospheric precipitation were the main sources of nitrogen pollution in Caohai.Combined with isotopic characteristic values,the analysis of nitrogen migration and transformation in water showed that nitrification occurred to a certain extent in wet season,but the reaction was weak in dry season,and denitrification did not occur in both wet and dry seasons.(5)Based on the above results,the nitrogen cycle model map of Caohai wetland in wet and dry seasons was preliminarily constructed.