Effect of root death and decay on dissipation of polycyclic aromatic hydrocarbons in the rhizosphere of yellow sweet clover and tall fescue

A 12-mo greenhouse study was conducted to evaluate the contribution of root death and decay on the dissipation of polycyclic aromatic hydrocarbons (PAHs) in rhizosphere soil. The contaminated soil was previously treated by land-farming, but residual PAHs remained after treatment. Tall fescue (Festuca arundinacea Schreb.) and yellow sweet clover (Melilotus officinalis Lam.) were the target plants. To specifically evaluate the effect of root decay on contaminant dissipation, plants were treated with glyphosate, a broad spectrum herbicide, to induce root decay. Although tall fescue treatments had the highest root and shoot biomass and root surface area, this plant did not result in the highest contaminant degradation rates. Significant differences were noted between treatments for seven PAHs, with the active yellow sweet clover resulting in 60 to 75% degradation of these compounds. Induced root death and decay did not produce a significant enhancement of PAH degradation. The PAH microbial degrader populations in the vegetated treatments were more than 100 times greater than those in the unvegetated control. The phospholipid fatty acid (PLFA) structural group profile shifted over the growing period, indicating a change in the community structure. In conclusion, phytoremediation was shown to be an effective polishing tool for PAH-affected soil previously subjected to biological treatment.