Abstract: Red soil from Chenzhou of Hunan and brown soil from Gongyi of Henan were collected and treated with Cu(NO₃)₂, Zn(NO₃)₂ or Cd(NO₃)₂, respectively for 2 weeks,and Rhizobium fredii strain HN01 was inoculated.Sequential extraction method was employed to investigate the forms of Cu, Zn and Cd in the examined soils with the absence and presence of rhizobia. The results showed that after inoculation,the total amount of solid bound Zn decreased 10%,and the amount of Zn associated with carbonate, Mn oxides,and organic matter fraction decreased 9～26%. No significant change was observed for the total amount of Zn combined with the solid phase of red soil in the presence of rhizobia. However, the amount of specifically adsorbed and Mn oxides bound Zn decreased, while the amount of exchangeable Zn increased. Inoculation of rhizobia depressed the release of Cu to the soil solution,and increased the total amount of Cu associated with the solid phase of brown soil. The increase of the amount of exchangeable Cu and of the Cu in fractions of carbonate, Mn oxides and organic matter ranged from 20% to 54%. There was no significant change for the level of Cd in the solution in both soils after rhizobia inoculation. The amount of exchangeable and organic Cd increased 22% and 11%, while the specifically adsorbed,and Mn oxides bound Mn decreased 14% and 29%, respectively. The different influence of rhizobia on the morphological distribution of test heavy metals in two soils was mainly ascribed to the soil pHchanges.

Key words: Soil, Rhizobia, Copper, Zinc, Cadmium, Fractionation