The Relationship Between Cd and Pb Forms and Their Availability to Rape in Major Soils of Hebei Province
河北主要土壤中Cd、Pb形态与油菜有效性的关系

Today, the problem of environmental pollution is becoming more and more serious. Specially, contamination of heavy metal has received increased attention with regard to their accumulation in soils and their potential to permanently damage plants, animals, and microorganisms. The effect of heavy metals on soil ecology is not controlled by their total amount; rather, it is determined by their chemical forms. Many studies have showed that the main form of heavy metals in soil that can be taken up by plants is the exchangeable form. Therefore, understanding the characteristics of heavy metals in soil is critical in order to develop a pollution index that can be used to evaluate the extent of heavy metal pollution in soil. The current study adopted the successive extraction method to investigate the characteristics of the chemical forms of cadmium (Cd) and lead (Pb) in the major soil types of Hebei Province. Also studied were the relationships between the forms of Cd and Pb forms and dry weight of the edible fraction of rape as well as the extent of metal accumulation in rape. The goal of the study was to identify the form(s) of heavy metals that can be closely related to their bioavailability as the more accurate index for the evaluation heavy metal pollution in soil. Two typical soil types (alluvial meadow soil, meadow cinnamon soil) of Hebei province were selected for the potting experiments and rape was used as a representative crop. There were five treatments with Pb concentrations of 0, 100, 300, 500, and 1000mg/kg, Cd treated concentration were 0, 1, 5, 10, and 50mg/kg. Every treatment was replicated three times. Each pot was filled with 5kg of soil. Rape grew 50 days. Plant samples were digested with HNO 3-HCLO 4. Soil samples were fractionated into the following 5 forms: exchangeable, carbonate-bound, Fe-Mn oxide-bound, organic-bound and residual, according to the modified extraction method of Zhu Wan-yan. All solution samples were analyzed for Cd and Pb using the flame atomic absorption spectrophotometry and the results were rectified using Cd and pb standards of the National Environmental Protection Bureau. The study clearly established the distribution characteristics of the various Cd and Pb forms in two soils investigated. We found that as the concentrations of added Cd and Pb increased, the exchangeable and carbonate-bound forms increased while the organic and residual forms decreased. In soils that were highly polluted with heavy metals, Cd mostly presented as exchangeable and carbonate-bound forms whereas Pb existed mainly in the carbonate-and Fe-Me oxide-bound forms. However, in less polluted soils, most of Cd was in the residual and the organic-bound forms while Pb were present in Fe-Mn oxide-bound and residual forms. This study also clarified the relationship between the chemical forms of Cd and Pb and their availability to rape. We found that the concentration of total and all forms of Cd and Pb in both soils were negatively corre