Soil bioaugmentation by free and immobilized bacteria to reduce potentially phytoavailable cadmium

Soil bioaugmentation was performed in soil pots to reduce the cadmium potentially available for plants. A Bacillus sp. (isolate ZAN-044) and a Streptomyces sp. (isolate R25) were compared, just as the inoculation technique, i.e., inoculum size, free or immobilized cells. After 3 weeks of a batch incubation, the potentially phytoavailable Cd was reduced, at the maximum, to a factor 14.1 and 4.3 with Bacillus sp. ZAN-044 and Streptomyces sp. R25, respectively. The two bacteria survived and colonized the soil. The immobilization technique did not improve the cell survival in the bioaugmented soil. The potentially phytoavailable Cd was positively (r(2)=+0.73) or negatively correlated (r(2)=-0.78) to the cell concentration in the sterilized soil bioaugmented with Bacillus sp. ZAN-044 or Streptomyces sp. R25, respectively. The major effect upon the phytoavailable Cd was the microorganism used and, to a lesser extent, the inoculum size and the culture technique.