| A large area exceeding soil quality standards for heavy metals in Southwestern and South-central China has been identified previously reported on the National Soil Pollution Survey Bulletin, and was considered as risks for soil quality and food safety.Yunnan province is located in the far southwest of China containing a number of parent materials types of soils. Heavy metals in soil profiles linking different soil parent materials have different geochemical behavior under the combined action of the southeast and the southwest monsoon.To thoroughly evaluate the risk of heavy metals in this region, seven soil profiles on the carbonate, shale and alluvium parent material were conducted in central Yunnan province, China, and the level of total concentrations and the fraction of speciation analysis of heavy metals was investigated in soil profiles. The main factors controlling heavy metal geochemical behavior were explored in weathering and soil formation of different soil parent materials; the potential ecological risks of heavy metals in area with different soil parent materials were discussed.Compared the ratio of different parent materials elements content over parent rock average can draw a conclusion that K, Al, Fe, Si are enriched, but Na, Ca, Mg are obvious lost in carbonate soil profile and the secondary enrichment happened to the As, Cd,Hg, Pb, Se in weathering process. During the shale weathering process, major elements are enriched(such as Al, Fe, Si), whereas K, Na, Ca, Mg are depleted. Heavy metals As, Pb, Hg, Se are enriched compare with Cd in shale soil profile. In sharp contrast to Na, Si depleted, K, Ca, Mg, Al, Fe are enriched during alluvium weathering process. The characteristics of As, Cd,Hg, Pb are in common with those in carbonate soil profile.In hot and humid climate, Na is lost due to the strong chemical weathering and pedogenesis, but Al, Fe are enriched regardless of soil parent material. Shale ranked as presenting the greatest the loss of Na, followed by alluvium, shale; carbonates was ranked first followed by alluvium and shale in degree enrichment of Al, Fe.The enrichment and depletion of heavy metals elements are not same in different parent materials. The degree environment of As, Cd,Hg, Pb in carbonates soil profile are obvious greater than alluvium soil profile. There are differences for degree enrichment of different heavy metals in same parent material or same pedogenic processes. Heavy metals enrich in the order, respectively, As>Pb>Hg>Cd in carbonates soil profile; Hg>Pb>As>Cd in shale soil profile; Cd>As=Hg>Pb alluvium soil profile.As, Pb in carbonates, shale, alluvium soil profiles were dominated by residual, next is humic acid and Fe-Mn oxidation. The percentage of carbonate, humic acid and Fe-Mn oxidation are higher than other fractions of Cd. The main chemical speciation of Se and Hg in soil is humic acid, strong organic and Fe-Mn oxidation.The total content of metal, pH and Corg takes an important role on As, Cd, Hg, Pb, Se chemical speciation(such as ion-exchange, carbonate, humic acid and Fe-Mn oxidation) in different soil profile. However, the water-soluble may be controlled by environmental conditions, irrelevant to parent material.The water-soluble and ion-exchange of Cd is enriched at depth of 0-20 cm in carbonates, shale, alluvium soil profiles which can be directly absorbed by plants. The findings suggest that environmental conditions of soil play a critical role in ecological risk of Cd. Thus, more studies are needed to better understand a linkage between the parent materials of soil, different soil-forming processes and the potential risk of Cd under various geographic conditions, which is the key for the evaluating soil quality and food safety. From conclusion we can speculate that the order of excessive risk of Cd in three parent materials is: carbonate >alluvium >shale. For shale and alluvium areas, the risk of As is still need evaluate in spite the percentage of water-soluble and total content of As is low. |
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