Bioavailability Of Heavy Metals In Paddy Soils

Heavy metals are widespread in soil,which is difficult to degrade,easy to accumulate,and have strong biotoxicity,and pose potential threats to the safety production of agricultural products,especially rice.The migration and transformation of heavy metals in the soil–rice system have become a key topic of concern for researchers and government departments in recent years.Currently,there are different interpretations of whether the heavy metal content in the soil is linked to heavy metal content in rice.Most studies believe that the rice heavy metals content is significantly affected by the soil heavy metals bioavailability,and the soil heavy metal bioavailability is related to the occurrence state in the soil.However,there is currently a lack of indicators for the bioavailability of heavy metals in rice soil,and relevant research works are the foundation and necessary to ensure the quality and safety of rice.A certain mining area in Zhejiang Province was selected as the subject of research for this study,and the corresponding content of roots and heavy metals in rice was measured.This study using the chemical extraction method to measure the content and occurrence state of heavy metals such as As,Cd,Cr,Cu,Pb,Ni,and Zn in rice soil.Then combined with the root-soil morphology,the enrichment coefficient method,the biological activity coefficient method,and the RAC risk evaluation coding method was used to analyze the bioavailability of the root-soil,and the regression analysis method is used to evaluate the bioavailability of soil heavy metals and propose indicators to characterize the bioavailability.Furthermore,for the heavy metal Cd which has high bioavailability and strong hazard,the variation law of the occurrence state of Cd in the soil on cross-sections within the study area was studied.This study provides an important benchmark for the rational use of agricultural land with potential risks of heavy metals pollution,particularly agricultural land in mining areas.The main conclusions are as follows:（1）The average values of heavy metals As,Cd,Cu,Pb,and Zn in the root-soil were higher than the background values of soil heavy metals in Zhejiang Province,while the average values of Cr and Ni were lower than the background values.The heavy metal content of Cd in some rice exceeded the contaminant limits in food,whereas the content of As,Cr and Pb was below the limits.（2）The order of heavy metal enrichment capacity is Cd>Zn>Cu>Ni>As>Cr>Pb,which suggests the absorption capacity of rice to different heavy metals in the soil is different.And there is some difference in the bioavailability of heavy metals based on the bioactivity coefficient method and the RAC risk assessment coding method.Among them,soil OM,pH,and CEC are significantly correlated with rice As enrichment coefficients,OM is significantly correlated with rice Cr enrichment coefficient,and soil CEC is significantly correlated with Zn.（3）The one-step extraction method showed that the leaching ability of different extracts on root-soil Cu,Ni,Zn was acidic extractant（HNO₃,HCl）>complexing agents（DTPA,EDTA）>neutral salts（Ca Cl₂,NH4OAc）.The extractants with the strongest extraction capacity for As,Cd,Cr,Cu,Pb,Ni and Zn were HNO₃,DTPA/HNO₃,HNO₃/Ca Cl₂,HNO₃,HNO₃,HNO₃,HNO₃ and the extractants with the weakest extraction capacity were NH₄OAc/Ca Cl₂,NH₄OAc,DTPA/NH₄OAc,Ca Cl₂,NH₄OAc,Ca Cl₂,NH₄OAc,respectively.（4）The BCR extraction method shows that the main occurrence states of the different heavy metals in the root soils are quite different.The main occurrence states of As,Cr,Ni,and Zn mainly occur in the residual state,Cd mainly occurs in weak acid extraction state and reducible state,Cu mainly occurs in the oxidizable state,and Pb mainly occurs in the reducible state.（5）The Cd content of most soil profiles in the study area decreased with increasing depth of the profile,and the enrichment of the surface soil was obvious.The soil content of different depth profiles in the study area was dominated by ACI-Cd,and the overall percentage decreased gradually with increasing depth（6）There is no significant correlation between the content of heavy metals Cr,Cu,Pb,Ni,and Zn in rice and the corresponding heavy metals content and occurrence states in soil.No indicators of their bioavailability were identified in this study.Based on soil heavy metals content,one-step extraction state content,and BCR extraction state content,linear and non-linear regression equations of rice heavy metal As and Cd content were established respectively.Of all regression models,the coefficient of determination of the cubic regression equation was the most significant.The DTPA extractable As content can be used as an index to characterize the soil As bioavailability.The soil Cd content,Ca Cl₂extractable Cd content and reducible Cd content can all indicate the soil Cd bioavailability.