Increased Magnesium Adsorption onto Colloids in Two Variable-Charge Soils in the Presence of Bacillus subtilis and Pseudomonas fluorescens

The adsorption of Mg2+ onto soil colloids from a Ultisol and an Oxisol in the presence of Pseudomonas fluorescens and Bacillus subtilis was studied in a batch adsorption experiment as a function of the Mg2+ concentration and bacteria/soil colloid ratio. The results indicated that Mg2+ adsorption was enhanced due to P. fluorescens and B. subtilis loading on soil colloids. The larger the amount of bacteria that were adhered to colloids, the greater the increase in Mg2+ adsorption. More bacteria were adhered to the Oxisol due to its higher contents of iron and aluminum oxides, and thus, the enhancement in Mg2+ adsorption induced by bacteria was greater than that achieved with the Ultisol. Both soils had a greater adhesion capacity for B. subtilis than for P. fluorescens, and therefore, B. subtilis was responsible for a greater enhancement of Mg2+ adsorption than P. fluorescens. When the concentration of B. subtilis was 6 mg L-1, the adsorption of Mg2+ on the Ultisol and Oxisol was 2.51- and 2.18-times that of the bulk soil without bacteria. At the same concentration of P. fluorescens, the adsorption of Mg2+ on the Ultisol and Oxisol was 1.54- and 1.92-times that of the bulk soil without bacteria. The zeta potential of the Ultisol and Oxisol changed from positive to negative or became more negative after the two bacteria were adhered onto soil surfaces at pH values from 3 to 8. The increase in Mg2+ adsorption was significantly correlated with the changes in zeta potential of soil colloids induced by the adhesion of bacteria. It was concluded that bacterial adhesion increased the negative surface charge in variable-charge soils, and thus, enhanced the adsorption of Mg2+ by the soils due to the increase in electrostatic interaction between Mg2+ and soil colloids.