Effect Of Rhizobia On The Speciations Of Cu,Zn And Cd In Red Earth And Cinnamon Soil

The effect of Rhizobium fredii HNOl on the speciation of Cu, Zn and Cd was studied in two China soils. First we discussed the fixation of Cu, Zn and Cd by soils and the speciation of immobilized heavy metal when the soils adsorbed the studied bacterium. Second we studied the effect of inoculating Rhizobium fredii HNOl on the Cu, Zn and Cd speications in the soils. The main results were shown as follows:1. Compared with the control, the fixation of Cu by red soil increased 105 mg/kg by 28% when the soil adsorbed Rhizobia. Adsorbing Rhizobia byred soil increased readily soluble and exchangeable Cu(Exc-Cu),specially adsorbed and other weakly bound Cu(Spe-Cu), Cu bound to Mn oxides(MnO-Cu) and Cu bound to organic matter (O.M-Cu) by 22.8-123% in the soil. The order of speciations of Cu changed in the presence of Rhizobia in red soil. The comparison of adsorbing with non-adsorbing Rhizobia showed no remarkable difference in content of total Cu in cinnamon soil,but adsorbing Rhizobia decreased Exc-Cu and increased Spe-Cu and MnO-Cu markedly in cinnamon soil.This indicated adsorbing Rhizobia reduced the mobility of Cu in cinnamon soil.The total content of Zn are 16.% higher by red soil adsorbing Rhizobia than non-adsorbing Rhizobia. When treated with Rhizobia, Exc-Zn increased 33.6mg/kg,accounting for 72% of total Zn increase. It appeared no difference in the fixation and speciations of Zn between adsorbing and non-adsorbing Rhizobia by cinnamon soil.Compared with non-adsorbing, adsorbing Rhizobia increased Cd adsorption by 27.7% by red soil. When treated with Rhizobia, Exc-Cd and Spe-Cd increased by 25.2% and 95.5% respectively in red soil. As well as Zn, there are no observable difference in the fixation and speciations of Cd between the treatment and the control in cinnamon soil.2. Added Rhizobia increased the amount of negative charge and metallo-ion specific adsorbing sites of solid compositions such as Mn oxides and organic matter etc. The influences of adsorbing Rhizobia by soil on the speciation ofheavy metal varied with the metal and the soil and were closely related to the mechanisms of metal fixation by soils. The Rhizobia made no effects on the fixation and speciation of heavy metal rooted in soil while the principle mechanism of metal-binding by soil was precipitation.3. Inoculating Rhizobia enhanced the dissolution and desorption of Zn in cinnamon soil. The dissolution of Zn are 10% higher in cinnamon soil inoculated with Rhizobia than non-inoculated and Spe-Zn, MnO-Zn and O.M-Zn decreased by 9.2-26.0% in the presence of Rhizobia. The comprison of inoculating with non-inoculating Rhizobia showed no evident distinction in total Zn, but inoculating Rhizobia decreased Spe-Zn and MnO-Znand increased Exc-Zn in red soil. Inoculating Rhizobia inhibited the Cu dissolution and desorption in cinnamon soil. The content of total Cu increased 73 mg/kg when the cinnamon soil was inoculated with Rhizobia. Compared with non-inoculating Rhizobia,inoculating Rhizobia showed 20.2-53.8% increase of Exc-Cu, Spe-Cu, MnO-Cu and O.M-Cu in cinnamon soil. It appeared that no observable difference in the Cu forms bewteen inoculating and non-inoculating Rhizobia in red soil. Inoculating Rhizobia didn't enhance or inhibited the dissolution and desorption of Cd in two soils,but changed the order of Cd speciations in red soil. Exc-Cd and O.M-Cd increased by 21.7% and 10.8% respectively, but Spe-Cd and MnO-Cd decreased 13.6-29.4% in the presence of Rhizobia in red soil. It appeared no remarkedly effect of inoculating Rhizobiaon on the Cd speciations in cinnamon soil.4. The effect of inoculating Rhizobia on the speciations of heavy metal varied with the metal and the soil. The Rhizobia have dual effects on the heavy metal in soils. On one hand, the organic components facilitated the binding of heavy metal, on the other hand, the organic acids secreted by the bacteria lead to the dissolution and desorption of heavy metal. The processes of dissolution and...