Soil samples were collected at an altitude of 500, 1,060, 1,500, 1,950, 2,400 and 3,100m, respectively, from Shennongjia, a forest reserve in Hubei province (central China). Their corresponding pHs were 5.50, 4.91, 5.64, 5.28, 5.49 and 4.60. By using a plant trap method, a total of 25 soybean rhizobia were isolated from the soil above an altitude of 1,500 m and all identified to be Sinorhizobium fredii. Their genetic biodiversity was characterized by 16S-23S rDNA internally transcribed spacer (ITS) region polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and random amplification DNA (RAPD) analysis. All the tested strains produced a 2.1 kb 16S-23S rDNA ITS fragment. After digestion with three restriction endonucleases (HaeⅢ, MspI and CfoI), respectively, great variations in 16S-23S rDNA ITS PCR-RFLP patterns were observed. The tested strains could be differentiated into 11 ITS genotypes. The genotypes of rhizobia were not related to geographical location. Twelve primers were applied to RAPD analysis and a dendrogram was obtained, showing that all the strains (including reference strain S. fredii USDA205) were divided into two diverging groups. Moreover, each group could be further divided into two subgroups. Both RAPD and 16S-23S rDNA ITS PCR-RFLP analysis indicated that a high degree of genetic diversity existed among S. fredii strains isolated from Shennongjia virgin soils. Since Shennongjia is an unexploited forest region in central China and the gene centre of soybean is located in China, the symbiotic genes harboured by these strains may be of great importance and the rich diversity of these strains might contribute to the adaptation of soybean to an alpine environment.

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.

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.