Plant-assisted bioremediation as an effectiveness strategy to remediate a historically PCB and heavy metal-contaminated area in Southern Italy

Plant-based clean up technologies are gaining popularity as sustainable solutions to contaminated soil remediation. In particular, plant-assisted bioremediation or phyto-assisted bioremediaton exploits the synergistic action between plant root system and natural microorganisms (bacteria and fungi) to remove, convert or contain toxic substances in soil and water. It relies on the use of a selected appropriate plant for stimulating in the rhizosphere (e.g. through root exudates production, oxygen transport) the biodegradation activity of natural soil microorganisms. In this context, a poplar-assisted bioremediation strategy has been applying for three years to a multi-contaminated (PCBs and heavy metals) area in Southern Italy using a specific poplar clone (Monviso). It was chosen thanks to its capabilities previous tested for promoting hexachlorocyclohexane degradation. At selected times (0, 420, 900 days) PCB and heavy metal (HMs: V, Cr, Sn, Pb) concentrations were assessed on soil samples at different depths and distance from tree trunks inside some contaminated plots. Similarly, microbial analyses were performed on soil samples to assess total microbial abundance, cell viability, dehydrogenase activity and the phylogenetic composition of the autochthonous microbial community. Three years after the poplar planting a significant decrease in PCB and HMs concentrations was observed. Currently, the values of all PCBs detected are under the Italian legislation limits in the plots investigated. The microbiological analysis show an overall improvement in soil quality in terms of an increase in microbial abundance, cell viability and organic carbon content in the rhizosphere soil samples. Moreover, the phylogenetic analysis of the microbial community showed a higher percentage of Bacteria in the rhizosphere than in the bulk soil. In particular, a significant increase in Actinobacteria and Alpha-Gamma-Proteobacteria, which include several speciesable to degrade PCBs, was observed. Overall results show that the poplar-assisted bioremediation strategy was able to promote both the persistent organic contaminant degradation and the phytostabilization of the inorganic ones.