Effect of inoculant density, formulation, dispersion and soil nutrient amendment on the removal of carbofuran residues from contaminated soil

Our study was aimed at investigating some factors affecting the use of a carbofuran-degrading bacterium as an inoculant to remove carbofuran residues from contaminated soil. Soil samples inoculated with a liquid cell suspension of strain C28 to give an initial density ranging from 103 to 107 cfu g−1 (dry weight of soil) significantly improved pesticide removal. When the soil was inoculated with a granular formulation of strain C28 to give theoretical cell densities of 5 × 103, 5 × 104 and 5 × 105 cfu g−1 (dry wt of soil), a significant increase in the rate of carbofuran degradation, as compared to uninoculated samples, was seen only for the highest inoculum size. The rate of carbofuran degradation in non-inoculated samples was significantly reduced by chloroform fumigation, but inoculation of natural and chloroform-fumigated soil with equivalent cell densities of strain C28 in both liquid and granular formulation led to a significant increase in the rate of carbofuran breakdown. The effect of soil glucose amendment on pesticide degradation by strain C28, in a liquid or solid formulation, depended on the concentration of the sugar and on inoculant density. A low concentration of glucose significantly reduced the lag phase before pesticide degradation by a low density inoculant. A high concentration, however, led to a significant reduction of the degradation rate for all inoculum sizes. Mixing the siol after inoculation was essential to obtain rapid degradation of the pesticide. When theoretically-equivalent cell densities were introduced in 0.1 and 1 g of microgranules, no significant difference in the rate of pesticide degradation was observed in these experiments.