Experiments were conducted to study the adsorption of Cd on two soil colloids (red soil and yellowbrown soil) and three variable-charge minerals (goethite, noncrystalline Fe oxide and kaolin) in the absence and presence of rhizobia. The tested strain Rhizobium fredll C6, tolerant to 0.8 mmol L-1 Cd, was selected from 30 rhizobial strains. Results showed that the isotherms for the adsorption of Cd by examined soil colloids and minerals in the presence of rhizobia could be described by Langmuir equation. Within the range of the numbers of rhizobial ceils studied, the amount of Cd adsorbed by each system increased with increasing rhizobial cells. Greater increases for the adsorption of Cd were found in red soil and kaolin systems. Rhizobia influence on the adsorption of Cd by examined soil colloids and minerals was different from that on the adsorption of Cu. The presence of rhizobia increased the adsorption affinity of soil colloids and minerals for Cd, particularly for the goethite and kaolin systems. The discrepancies in the influence of rhizobia on the adsorbabliity and affinity of selected soil colloids and minerals for Cd suggested the different interactions of rhizobia with various soll components. It is assumed that bacterial biomass plays an important role in controlling the mobility and bioavallability of Cd in soils with kaolinite and goethite as the major colloidal components, such as in variablecharge soil.

Red soil and cinnamon soil were collected from Chenzhou of Hunan and Gongyi of Henan, respectively. Soils were treated with Cu(NO3)2, Zn(NO3)2 or Cd(NO3)2, respectively, for two weeks. Rhizobium fredii strain HN01 was inoculated into the two soils polluted with three heavy metals. Sequential extraction method was employed to investigate the forms of copper (Cu), zinc (Zn), and cadmium (Ca) in the examined soils with the absence and presence of rhizobia. Results showed that the total amount of solid-bound Zn decreased 10% after the inoculation. The decrease for the amount of Zn associated with carbonate, manganese (Mn) oxides, and organic matter fraction was from 9 to 26%. No significant change was observed for the total amount of Zn combined with 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 solid phase in cinnamon soil. The increase for the amount of exchangeable Cu and 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 Cd in the fractions of exchangeable and organic increased 22 and 11%, while that in the fractions of specific and Mn oxides decreased 14 and 29%, respectively. The different influence of rhizobia on the distribution of three heavy metals in two soils was mainly ascribed to the growth status and pH changes exerted by the metabolites of rhizobia. These data are helpful for the understanding of the chemical behavior and biogeochemical cycle of heavy metals affected by microorganisms in soil environment, which is fundamental for heavy metal bioremediation.

Heavy metal pollution presents a major hazard to the soil environment. Studies have shown that the activities of a variety of soil enzymes are inhibited by heavy metals. However, little information is available concerning the effect of heavy metals on the activity of enzymes immobilized by different soil constituents. The main objective of this work was to investigate the effects of copper on the activity and kinetic properties of acid phosphatase both free and immobilized on two variable-charge soil clays and the minerals kaolin, goethite and manganese oxide. The effect of different forms of copper on enzyme activity was also examined. In the presence of copper chloride, the activity of free and immobilized enzymes was inhibited at copper concentrations of 0.005-0.8 mM at pH 5.0 and inhibition increased at pH 6.0. The inhibitory effect of copper chloride was greater on the enzymes bound by the two soil clays and kaolin than those by goethite and MnO2. Addition of copper chloride decreased both the Km values and the Vmax/Km ratios of free and all forms of immobilized enzymes, and showed mixed type inhibition kinetics. Comparing the effect of different forms of Cu, the residual activities of free enzyme and soil clay-enzyme and kaolin-enzyme complexes were higher when copper citrate was used than with copper chloride. The reverse was true for the enzymes immobilized on goethite and MnO2. These results indicate that the inhibition by Cu of enzymes immobilized on soil components are influenced by the properties of the adsorbent and the form of Cu, as well as pH.