Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous, recalcitrant, and potentially carcinogenic pollutants. Plants and their associated rhizosphere microbes can promote PAH dissipation, off ering an economic and ecologically attractive remediation technique. Th is study focused on the eff ects of diff erent types of vegetation on PAH removal and on the interaction between the plants and their associated microorganisms. Aged PAH-polluted soil with a total PAH level of 753 mg kg −1 soil dry weight was planted with 18 plant species representing eight families. Th e levels of 17 soil PAHs were monitored over 14 mo. Th e size of soil microbial populations of PAH degraders was also monitored. Planting signifi cantly enhanced the dissipation rates of all PAHs within the fi rst 7 mo, but this eff ect was not signifi cant after 14 mo. Although the extent of removal of lower-molecular-weight PAHs was similar for planted and unplanted control soils after 14 mo, the total mass of fi ve-and six-ring PAHs removed was signifi cantly greater in planted soils at the 7-and 14-mo sampling points. Poaceae (grasses) were the most eff ective of the families tested, and perennial ryegrass was the most eff ective species; after 14 mo, soils planted with perennial ryegrass contained 30% of the initial total PAH concentration (compared with 51% of the initial concentrations in unplanted control soil). Although the presence of some plant species led to higher populations of PAH degraders, there was no correlation across plant species between PAH dissipation and the size of the PAH-degrading population. Research is needed to understand diff erences among plant families for stimulating PAH dissipation.

Phytoremediation offers an ecologically and economically attractive remediation technique for soils contaminated with polycyclic aromatic hydrocarbons (PAHs). In addition to the choice of plant species, agronomic practices may affect the efficiency of PAH phytoremediation. Inorganic nutrient amendments may stimulate plant and microbial growth, and clipping aboveground biomass might stimulate root turnover, which has been associated with increases in soil microbial populations. To assess the influence of fertilization and clipping on PAH dissipation in a nutrient-poor, aged PAH-contaminated soil, a 14-mo phytoremediation study was conducted using perennial ryegrass (Lolium perenne) as a model species. Six soil treatments were performed in replicate: unplanted; unplanted and fertilized; planted; planted and fertilized; planted and clipped; and planted, clipped, and fertilized. Plant growth, soil PAH concentrations, and the concentrations of total and PAH-degrading microorganisms were measured after 7 and 14 mo. Overall, planting (with nearly 80% reduction in total PAHs) and planting + clipping (76% reduction in total PAHs) were the most effective treatments for increased PAH dissipation after 14 mo. Fertilization greatly stimulated plant and total microbial growth, but negatively affected PAH dissipation (29% reduction in total PAHs). Furthermore, unplanted and fertilized soils revealed a similar negative impact (25% reduction) on PAH dissipation after 14 mo. Clipping did not directly affect PAH dissipation, but when combined with fertilization (61% reduction in total PAHs), appeared to mitigate the negative impact of fertilization on PAH dissipation. Therefore, fertilization and clipping may be included in phytoremediation design strategies, as their combined effect stimulates plant growth while not affecting PAH dissipation.