Microorganism and Medicago sativa synergic effects on PCB degradation in a contaminated soil

Polychlorinated biphenyls (PCBs) are organic hydrophobic persistent pollutants which are found as diffuse contaminants both in soil and sediment. Their degradation occurs mainly by biotic aerobic and anaerobic processes mediated by microorganisms. Their degradative activity can be promoted in soil by plant occurrence, because plant roots releasing exudates influence directly and indirectly PCB biodegradation. The effectiveness of PCB degradation is based on the use of toxicant tolerant plant species. Moreover, compost can be added to promote both plant and microbial activity. In order to better investigate the relationships between plant roots and natural microbial populations, soil samples from a PCB contaminated site, localized near Taranto, were used for performing degradation experiments in microcosms. The species Medicago sativa and compost derived from municipal solid waste were differently added to soil samples. The experimental set up was maintained in a greenhouse for about 8 months under temperature and water controlled conditions. Microbiological and chemical analysis were carried out at different times (0 , 4 and 8 months) in order to assess the changing in structure and functioning of microbial populations related to PCB degradation. The overall results show the complexity of PCB degradation processes and that each treatment (e.g. plant occurrence/absence or presence/absence of compost) acts differently on the degradation of the various PCBs analyzed, promoting the decrease of some congeners and the formation and accumulation of others.