Magnetic Properties Of Farmland Soils In Typical Oasis In Northwest China And Their Response To Heavy Metal Pollution

Farmland soil is the natural base for crop growth,and its environmental quality is closely bound up with the quantity and quality of agricultural products,human health and the balance of rural eco-environment.However,due to the unreasonable farming way,the extensive use of agricultural inputs and the moving of pollution industries from urban to suburban and rural areas,farmland soil facing a big pollution risk.The land area of our country is vast,and a comprehensive,rapid and low-cost detection of farmland soil is currently needed,the environmental magnetism method has obvious advantage in the aspection of large area,rapid and economic monitoring for heavy metal.Therefore,102 farmland topsoil samples and 4 farmland soil profiles were sampled from Zhangye City in the northwest arid areas of China,the spatial distribution and vertical variation characteristics of magnetic minerals and heavy metals in farmland soil samples were analysed base on the environmental magnetism method and chemical method,and the response relationship between magnetic parameters and heavy metals were discussed in order to understand the feasibility of environmental magnetism method in farmland soil pollution monitoring.The main conclusions in this study are as follows:The main magnetic mineral of farmland soil samples is ferri-magnetic magnetite,the magnetic grain-size is between 0.2-1 ?m in equivalent diameter,and the domain states are mainly multi-domain(MD)and coarse-grained pseudo single-domain(PSD).Low frequency magnetic susceptibility(?lf)value of farmland soil samples ranged from 22.27×10-8 m3/kg to 188.36×10-8 m3/kg with the average value of 63.85×10-8 m3/kg,and overall the magnetic mineral content of farmland soil samples is low,but it have large spatial variation in space.The spatial distribution of magnetic parameters show that the magnetic mineral contents in Linze county is lower than in Ganzhou district,Minle county and Shandan county,and three relatively high-value areas present in central Ganzhou district,southwest side of Minle county and west side of Shandan county,respectively.High magnetic value in central Ganzhou district is mainly controlled by human activities,such as industrial production,while the high magnetic values in southwest side of Minle county and west side of Shandan county are more related to strong natural pedogenesis.The vertical variation of magnetic parameters show: in Linze profile,?lf is low and stable like a straight line,and most ?fd% is below 3%,Linze profile can be considered as a non-pollution background profile which have very weak pedogenesis;in Ganzhou profile,?lf have a significant increase at the depth of 0~26 cm,while which are low and stable at the depth of 26~100 cm,the magnetic enhancement is caused by anthropogenic input.The background magnetism of Shandan profile and Minle profile are stronger than Ganzhou profile and Linze profile.The results of multivariate statistical analysis between magnetic parameters and comprehensive pollution indexs(PN,RI)of heavy metal elements(V,Cr,Mn,Fe,Ni,Cu,Zn)in farmland soil show that HIRM can be used as a substitute index for natural heavy metal elements accumulation degree;?lf,?ARM,SIRM and SOFT can be used as substitute indexs for anthropognic heavy metal elements pollution degree.?lf,?ARM,SIRM and SOFT can effectively trap the farmland soil range which is polluted by heavy metals from the city.