Ecological Geochemistry Of Cadmium In Soils From A Typical High Geogenic Background Karst Area

The accumulation of heavy metals in soil-rice systems is an important environmental problem.Much attention has been paid to the anthropogenic soil pollution,but the study on the geogenic heavy metal pollution is relatively lacking.The bulletin of the national soil pollution survey shows that the metal elements in the soil in karst areas of southwest China are usually exceeding the standard,and the accumulation of heavy metals is related to geological processes.At present,the characteristics of heavy metal enrichment,formation mechanism of high geogenic background,ecological risk of pollution,bioavailability and migration process of heavy metals in farmland soil from high geogenic background karst area are still unclear.Global carbonate distribution area occupies 12%of the land area,our eountry is the largest country in the karst area in the world,by studying ecological geochemistry characteristics of cadmium in soil-rice system from the typical karst area in Guangxi,can be high for the geological background of the ecological risk assessment of soil heavy metal pollution,providing theoretical basis and the premise for cadmium pollution of soil remediation.In this paper,the paddy soils and rice from high geological background karst area in Guangxi were studied,and concentration of As,Cd,Cr,Cu,Ni,Pb and Zn were analyzed,the characteristics and control factors of heavy metals content in soil were discussed,the sources of heavy metals in soil and relationship between soil carbonate minerals and enrichment of heavy metals in soils,and the migration regularity of heavy metals in soil-rice system were analyzed.In addition,two extraction methods,CaCl2 and EDTA(Ethytlene Diamine Tetraacetic Acid),were used to analyze the bioavailability of metals in soil,so as to evaluate the ecological risk of soils from the high geogenic background karst area and its difference from the anthropogenic contaminated soil.This paper comes to the following conclusions:(1)The order of enrichment coefficient(soil/upper crust)of different heavy metals in soils from typical karst areas in Guangxi is Cd>Pb>As>Zn>Cr>Cu>Ni.The accumulation of heavy metals in soil has a significant inheritance of parent rocks(carbonate rocks),and the carbonate rocks have an initial Cd enrichment when formed.Soil Cd concentration was positively correlated with CaO,TFe2O3,Corg content and pH(p<0.01),and negatively correlated with soil K2O,Na2O and SiO2 content(p<0.01).According to the soil environmental quality agricultural land soil pollution risk control standard(GB15618-2018),the Cd concentration in the research area exceeds the standard seriously,as high as 57.2%.(2)Soil Cd content is closely related to carbonate species.δ13C indicates the ratio of primary and secondary carbonates,primarily primary(clastic)carbonates in the slope sediments and secondary carbonates in the soil.The lower the proportion of primary carbonate and the higher the proportion of secondary carbonate,the more negative δ13C,the more concentrated the soil Cd.(3)The Cd concentration of the whole rice plant is more abundant than that of the rhizosphere soil.Most of the Cd uptaked by rice(92%)is stored in the non-edible part(rhizome),and a very small part(8%)is stored in the grain.Soil with high Cd concentration does not necessarily have high Cd concentration in rice grain.There was a significant positive correlation between Cd concentration in rice and total Cd concentration in soil(p<0.05),a significant negative correlation between the transfer coefficient of Cd from soil to rice and the concentration of soil S and TFe2O3(p<0.01),a significant positive correlation between the translocation factor of Cd from rice root to grain and the concentration of soil Zn(p<0.01),and a significant negative correlation between the concentration of soil Corg and N(p<0.01).(4)The average concentration of CaCl2 extracted Cd in karst areas is 0.02mg·kg-1,which is much lower than the extracted concentration of soil Cd in anthropogenic polluted(industrial and mining)areas.Different from the linear relationship between CaCl2 extracted Cd concentration and the total concentration of soil Cd in anthropogenic polluted areas,there is no significant linear relationship between the CaCl2 extracted Cd concentration and the total concentration of soil Cd in karst areas,which is mainly controlled by pH.(5)Cd/Zn ratio of soil and extracted concentration affects the cadmium content in rice.Cd/Zn ratio in rice(0.02)between soil Cd/Zn ratio(0.007)and Cd/Zn ratio of extracted concentration(0.1).The ratio of Cd/Zn in rice was positively correlated with that in soil and available Cd/Zn(p<0.01).By reducing the soil available Cd/Zn ratio,can reduce the plants uptaked Cd/Zn ratio,and thus reduce Cd content of plants.