Addition of maize stalks and soybean oil to a historically PCB-contaminated soil: effect on degradation performance and indigenous microbiota

Objective of this study was to assess the single or combined effect of a plant oil and a lignocellulosic waste, namely soybean oil (SO) and maize stalks (MS), respectively, on resident microbiota and bioremediation performances of a soil historically contaminated by medium to highly chlorinated PCBs. Higher concentrations of both biphenyl- and chlorobenzoate-degrading cultivable bacteria were found in the MS-amended microcosms (MSM) than the non amended or SO-amended ones after 30 d incubation at 28°C. Fungal growth, instead, was strikingly stimulated in the microcosms that had undergone concomitant MS and SO supplementation (MS-SOM). Denaturing gradient gel electrophoresis analyses of 16S and 18S rRNA genes showed that both amendments promoted a remarkable increase in both bacterial and fungal biodiversity. The abundances of biphenyl-2,3-dioxygenase (bph) and that of catechol-2,3-dioxygenase (C230) genes in the non-amended contaminated soil were constant over time. Conversely, after 60 d incubation, bph and C230 abundances increased 2.8- and 61-fold in the MSM, respectively, and, in the MS-SOM, 1.4- and 46-fold, respectively, with respect to the zero time point. Although the overall PCB removal was not positively affected by the amendments, the concomitant presence of both MS and SO led to significantly higher depletions of hexa-, hepta-, octa- and nona-chlorinated congeners than in the non-amended microcosms (i.e. 24.6, 22, 20.5 and 9.5%, versus 19.4, 16.4, 14.7 and 6.1%, respectively). In all microcosms, PCB degradation was negatively correlated with hydrophobicity, organic matter/water partition coefficient, molecular weight and extent of chlorination of the pollutants with the notable exception of the MS-SOM ones where such a relationship was less stringent.