Removal of cadmium and polychlorinated biphenyls by clover and the associated microbial community in a long-term co-contaminated soil

Legumes such as clover are cost-effective and environmentally friendly components of strategies for remediating soils contaminated with heavy metals or organic pollutants. However, the mechanisms by which clover remediates co-contaminated soils are unclear. The present study explored the effects of phytoremediation by clover on pollutant removal and the microbial community in soil co-contaminated with cadmium (Cd) and polychlorinated biphenyls (PCBs). After 18 months of phytoremediation, Cd removal increased from 20.25 % in the control to 40.65 % in soil planted with clover, while PCB removal increased from 29.81 % to 60.02 %. High-throughput sequencing analysis showed that the relative abundances of the bacterial phylum Proteobacteria and the diazotrophic genus Rhizobium increased significantly after phytoremediation. Random forest analysis showed that bacterial and diazotrophic diversity significantly influenced Cd and PCB removal. Furthermore, co-occurrence network and correlation analyses revealed that Rhizobiales and Micromonosporales were the main bacteria associated with Cd removal, while Rhizobiales, Burkholderiales, and Xanthomonadales were identified as the main degraders of PCBs. PICRUSt functional prediction demonstrated that the gene bphC, which is related to PCB degradation, was significantly increased in the rhizosphere soil in the presence of clover. These results provide a better understanding for further studies of remediation efficiency by clover, rhizosphere microbial response and remediation mechanisms of co-contaminated soils under in situ conditions in the field.