Spatial Distribution And Risk Assessment Of Heavy Metals And Nitrogen And Phosphorus In Different Land Use Patterns In The Caohai Watershed Of Guizhou

Caohai is a complete and typical karst plateau wetland ecosystem in Guizhou,and it is one of the main habitat of Grusnigricollis in winter of China.With the problems of soil erosion and eutrophication in Caohai,the threat of terrestrial pollutants has been intensified,and the soil contaminants has became the primary environmental management problem.This study takes Caohai basin as the research object.The data of the land use of 2009,2014 and 2017,the heavy metals,nitrogen and phosphorus of surface soil were used,and the landscape pattern index method,the land use dynamic change model method,the ecological risk assessment method,the fuzzy comprehensive evaluation method were used to analyze the land use change in the Caohai Basin from 2009 to 2017,to reveal the distribution of soil heavy metals,TN and TP under different land use types,and to evaluate the potential ecological risk of soil heavy metals,the risk of nitrogen and phosphorus loss and its comprehensive ecological risk.The main conclusions are as follows:(1)During the period from 2009 to 2017,the cultivated land in Caohai basin changed the most,and the overall of land use type changed slow.The land use types change were analyzed using single type dynamic degree and the overall dynamic degree.The results showed that the dynamic degree of cultivated land,urban land,water area,forest land and grassland was-1.53%,2.93%,0.51%,4.26%and 1.28%,respectively.The cultivated land decreased by 6.37 km~2,and urban land,water area,forest land and grassland were added 1.34 km~2,0.84 km~2,3.25 km~2and 0.94 km~2,respectively.The overall dynamic degree was 1.31%,indicating that the land use change was slow.(2)Land use patterns affect the distribution of heavy metals,TN and TP in surface soils.Urban land and cultivated land,which are strongly influenced by human activities,have a high impact on the contents of heavy metals,TN and TP.The results showed that land use had significant effects on Cd,Zn,TN and TP content,but on Hg and As(P<0.05).There were no significant effects on the contents of Cu,Pb,Ni and Cr(P>0.05).Among them,Zn showed urban land>forest land>cultivated land>irrigation grassland,TP showed cultivated land>urban land>irrigation grassland>forest land,Cd and TN were both urban land>cultivated land>Woodland>Irrigation.In general,urban land and cultivated land,which are strongly influenced by human activities,have a higher impact on heavy metals,TN and TP loads than forest land and grassland.(3)The comprehensive ecological risk of soil heavy metals in Caohai basin is moderate ecological risk level,and Cd,Zn,Pb and Hg are the main soil heavy metal pollution factors.The potential ecological risk assessment of heavy metals in soil showed that Cd in all four land use types were heavily polluted,and Pb in urban land and woodland and Zn in urban land were moderately polluted.Non-polluting or light pollution;Hg is a moderate ecological risk in forest land and cultivated land.Cd is a strong ecological risk in urban land.Cd is a strong ecological risk in forest land,grassland and cultivated land,and the rest is mild.Ecological risk;RI is between 153.53-243.66,which is a medium ecological risk level.(4)The TN is high and TP is low in Caohai,and the spatial distribution of nitrate nitrogen and available phosphorus loss are significantly different.The contents of TN and TP in topsoil of Caohai basin were 2.48 g/kg and 0.22 g/kg,respectively.Which respectively were the first and fifth grades of national soil nutrient standards.The high and higher loss risk of soil nitrate nitrogen is mainly distributed in Zhengjiaying,Caohai,Yazitang and Xiangtang.The high loss of soil available phosphorus is mainly distributed from Caohai Village to Fumin Village.The low risk of nitrogen and available phosphorus is mainly distributed in the southwest,northwest and east of the basin.(5)The comprehensive risk of soil pollution in Caohai basin is at a moderate risk level.Among the 75 samples sampled,there were no high-risk samples,of which 9 were evaluated as low-risk,and 66 were evaluated as medium-risk,indicating that the comprehensive risk of soil pollution in Caohai basin was moderate level.