Study On Soil Element Distribution And Heavy Metal Pollution In Typical Sample Areas In Central And Northern Shaanxi

The central and northern parts of Shaanxi Province are known as an important production base for fruit industry and agriculture,where heavy metals have continuously accumulated in soil due to long-term use of pesticides,fertilizers and mulching films,etc.Previous studies mainly focused on surface pollution,but the root system of fruit trees is as deep as more than 2 m,and the quality of deep soil also has a significant impact on the quality of apples.In order to ascertain the distribution of soil elements and pollution of heavy metals in the central and northern parts of Shaanxi Province,and reveal the characteristics and laws of the south-north spatial variation and the cause and mechanism of heavy metal accumulation,this paper studies the soil quality of the surface soil and shallow loess-paleosol profiles in typical sample areas of the central and northern parts of Shaanxi Province.In fact,it can provide scientific basis for the prevention and control of heavy metal pollution in soil and the sustainable development of agriculture in the area,and is of great guiding significance for the development of high-quality fruit industry.In this paper,14 typical sample areas of surface（0～50 cm）soil in Qianxian county,Xunyi county,Yijun county and Qingjian county in the central and northern parts of Shaanxi Province,and 9 typical sample areas of shallow loess-paleosol profile（L0～L2）in Qianxian county,Xunyi county and Yijun county are studied,and experimental analysis is carried out on soil pH value,organic matters,size composition,mineral composition,CaCO3 content,magnetic susceptibility,soil microstructure,content of macroelements and microelements,content of heavy metals and their mode of occurrence.Based on experimental data,single-factor pollution index method,Nemerow integrated pollution index method and potential ecological risk index method is used to evaluate the pollution and potential ecological risk of heavy metals in surface soil at the depth of 0～20 cm and 20～50 cm in the study area and in shallow loess-paleosol profiles.At the same time,the impact of the physical and chemical properties,land utilization patterns,soil mineral composition and soil microstructure on the distribution of heavy metal elements and natural enrichment in soil of the study area is thoroughly explored by means of correlation analysis and independent-samples T test,and the controlling effect and mechanism of the climate on the natural enrichment of heavy metal elements are further revealed.Finally,the main understandings and conclusions are drawn as follows:（1）According to the background values of soil elements in Shaanxi Province,heavy metal elements（such as Cu,Zn,As,Pb,Ni and Cr）are accumulated to a certain extent in 14 surface soil sample areas.Specifically,the accumulation degree of heavy metal elements is slightly higher in Xunyi surface soil sample area while slightly lower in Qingjian surface soil sample area,and that in Qianxian county and Yijun county are in between.From the perspective of mode of occurrence of elements,Cr,Cu,Ni,Zn and other elements are dominated by residual form,while Cd and Pb are dominated by non-residual form,indicating that Cd and Pb have are highly bioactive and can easily be utilized by organisms.In addition,there is no significant difference in the distribution of heavy metal elements in various forms between the north and the south sample areas,but the content of element forms tends to increase as the total amount of heavy metals increases on the whole.（2）In the shallow loess-paleosol profiles of 9 typical sample areas,most of the elements such as heavy metal elements Cu,Zn,As,Pb,Ni and Cr and microelements V,Mn,Rb and Ba are characterized by a significantly higher content in Holocene S0 and late Pleistocene S1 paleosol than in loess and surface soil.The content of heavy metals shows that S0 and S1 paleosol is the stratum where heavy metals are the most enriched in the study area.（3）In general,pH value,organic content and clay content of surface soil have an impact on the content of heavy metals,but the impact is not significant.In the shallow loess-paleosol profiles,clay content and magnetic susceptibility have a significant positive correlation with the content of heavy metals,while CaCO3 content has a significant negative correlation with the content of heavy metals.In the study sample areas,colloidal particles,newly-formed ultrafine crystalline clay minerals and argillans in the shallow paleosol also play a very important role in the natural enrichment of heavy metals.（4）Climate plays an important role in the natural enrichment of soil heavy metals in loess-paleosol profiles.Under the conditions of annual average precipitation of 700～1000 mm and annual average temperature of about or slightly higher than 15℃,it helps the natural enrichment of heavy metal elements in paleosol.Under the temperate climate conditions of annual average precipitation of about or lower than 600 mm,there is generally no natural enrichment of heavy metals in soil.（5）According to the evaluation results of heavy metal pollution in surface soil at the depth of 0～20 cm and 20～50 cm in various sample areas,it can be found that the pollution level of heavy metals in surface soil at the depth of 20～50 cm is slightly lower than that at the depth of 0～20 cm,but the pollution degree is not lowered,indicating that the layer of surface soil at the depth of 20～50 cm is also influenced by human activities to a certain extent.The south-north spatial variation of heavy metal pollution in surface soil of the central and northern parts of Shaanxi Province is characterized in that the pollution degrees of heavy metals in surface soil are similar,the pollution degree in Xunyi surface soil sample area is slightly higher than that in Yijun county and Qianxian county,while that in Qingjian surface soil sample area is slightly lower than that in Yijun county and Qianxian county.Specifically,the integrated pollution degree in Xunyi,Yijun and Qianxian surface soil sample areas is classified as mild pollution,and the level of integrated potential ecological risk reaches moderate ecological risk.In addition,the integrated pollution degree in Qingjian surface soil sample area is classified as clean,and the level of integrated potential ecological risk reaches mild ecological risk.（6）According to the evaluation results of heavy metal pollution in the soil layer of shallow loess-paleosol profiles in 9 sample areas,it can be seen that single-factor pollution index,single-factor potential ecological risk index,integrated pollution index and integrated potential ecological risk index hit the peak in the stratum of paleosol,and the integrated pollution degree of paleosol in shallow loess-paleosol profiles of 9 samples areas is classified as mild pollution,but is slightly higher than the heavy metal pollution in surface soil,which is the outcome of the natural enrichment of natural metal elements.The south-north spatial variation of paleosol heavy metal pollution in shallow loess-paleosol profiles of typical sample areas in the central and northern parts of Shaanxi Province is characterized in that Yijun sample area is heavier than Qianxian county sample area,while Qianxian county sample area is heavier than Xunyi sample area,and the law of variation is characterized in that the enrichment of heavy metals increases or decreases as the precipitation increases or decreases.（7）The mechanism of natural enrichment and pollution of paleosol heavy metals in shallow loess-paleosol profiles of the study area is as follows:Take the warm and wet climate with the annual average precipitation of over 700 mm as the driving force→drive CaCO3 leaching and transporting in soil and lead to the preliminary relative enrichment of heavy metal elements→result in the decomposition and clayization of silicate minerals→form clay particles,clay mineral micrograins of colloidal size,colloid strips,argillans and other components with strong absorption→make soil have a strong absorption and absorb heavy metals in soil solution from atmospheric dust deposition,atmospheric precipitation,plant decomposition and other sources→generate the enrichment of heavy metals and lead to heavy metal pollution.