Heavy Metals In Soil-crop System From Typical High Geological Background Areas,Southwest China:Transfer Characteristics And Controlling Factors

Southwest China is the largest high geological background area of heavy metals in China.According to the previous studies,soils in Southwest China were anomalously enriched in heavy metal with high exceedance rate,which may pose ecological risk to food safety in the area.However,the ecological risk of heavy metals in soil is closely related to their sources.Geogenic heavy metals usually have lower bioavailability and ecological risk.However,the ecological risk and influence factor of soil heavy metal contamination in Southwest China is unclear,due to the lack of systematic study in this area,which is not conducive to develop appropriate management of the croplands and ensure the safety of agricultural products.In order to find out the transfer and enrichment characteristics,geochemical speciation,bioavailability and controlling factors of heavy metals in soil-crop system in Southwest China,the rock,vertical soil profile,rhizospheric soil and crop samples were collected from Hengxian County?carbonatite exposed areas?and Zhaotong City?Emei Mount basalt exposure areas?in this research.The total content of heavy metals in rock and crop samples,total and Ca Cl₂ extractable heavy metals,geochemical speciation,physical and chemical properties?p H,organic matter,texture,and mineral composition?of soil samples were analyzed.We applied multivariate statistical analysis approaches,such as correlation analysis,cluster analysis and stepwise regression,and the following conclusions were obtained.The contents of heavy metals in the soil of carbonatite area in Hengxian County were significantly higher than that in their parent rock,which indicating that the soil has undergone a strong secondary enrichment during the soil pedogenic process.The contents of Cu,Ni,Zn,Co and V in the soil of basalt area in Zhaotong City were similar to their parent rock.This indicates the inheritance to their parent rock.While,the contents of Cd,Cr and Hg were significantly higher than in their basalt parent rocks,which may due to secondary enrichment during the soil pedogenic process.The results of correlation analysis shows that the heavy metals are enriched due to the loss of Si,K,Na during weathering process in the carbonatite areas in Hengxian County.The iron and manganese nodules may be the main carriers of heavy metals in soil.Copper,Ni,Zn,Co and V in the soil of basalt exposed areas in Zhaotong City may mainly combine with Fe and Mn oxides,As,Hg and Pb were tend to be enriched in sulfide phase,while Cd may combined by phosphate.Hengxian County?carbonatite exposed areas?and Zhaotong City?Emei Mount basalt exposure areas?both have high contents of heavy metals in soil,with low bioavailability and low exceedance rate in crops.The bioconcentration factor?BCF?of heavy metals in crops in Hengxian carbonatite area and Zhaotong basalt area were significantly lower than other non-high geological background areas and anthropogenic pollution areas.The BCF varied among heavy metals and different areas.These indicates the enrichment of heavy metals in soil-crop system were co-affected by the chemical properties of elements,the physical and chemical properties of soil and the biological processes in different crops.Stepwise multiple regressions revealed that the weathering intensity of soil and the contents of Si,Al,Fe,K were the dominant factors that control the bioavailability of heavy metals in the carbonatite area of Hengxian County;while,the total contents of heavy metals in the soil were the main controlling factors of the bioavailability of Cu,Hg and Zn in the Zhaotong basalt area.The prediction equations with high accuracy?R²>0.5,P<0.01?for the bioconcentration factor?BCF?of As,Cd,Cr,Cu,Hg,Ni,and Zn in the carbonatite exposed areas of Hengxian County and Cu,Hg,and Zn in basalt exposed areas of Zhaotong City were established.