Phytoremediation of a PCB-contaminated soil by alfalfa and tall fescue single and mixed plants cultivation

Two plant species (tall fescue and alfalfa) grown alone and in combination was investigated to evaluate phytoremediation of polychlorinated biphenyl (PCB)-contaminated soil and the impact on the catabolic genes and soil enzyme activities in the rhizosphere. Surface soil was collected from Xiaoshan (a PCB-containing capacitors and transformers storage site). The phytoremediation experiment was carried out in an agricultural greenhouse experiment station at Huajiachi campus, Zhejiang University. Soil dressing method and fertilizers were applied to improve soil quality for the pot experiment. Soil dehydrogenase activity was measured spectrophotometrically by the reduction of 2, 3, 5-triphenylterazolium chloride (TTC) to triphenyl formazane (TPF) and catalase activity was determined by back-titrating residual H2O2 with KMnO4. Quantitative real-time PCR using SYBR green I was employed with the individual primer sets to determine the relative abundance of the biphenyl dioxygenase genes. Total bacterial numbers were determined by CFU counting and amplification of 16S rDNA. Planting treatment significantly enhanced bacterial numbers and PCB removal. The copy numbers of the bphA, bphD.1.B, bphD.2.A, and bphD.2.A/B genes, total bacteria counts, and dehydrogenase activity were the highest in mixed cropping soil, which indicated that tall fescue (forage grass) and alfalfa (legume) mixed cropping was most beneficial to soil bacteria, the potential PCB degraders, and enzyme activity. However, the highest removal of PCBs was found in tall fescue single plant cultivation, followed by combined plant cultivation, probably because tall fescue had greater biomass and could extract more PCBs from soil. Compared with nutrients amended unplanted control, the removals of tri-, tetra-, and penta-CBs in tall fescue single-planted and tri-CBs in combined planted soils were significantly enhanced (p < 0.01). The presence of vegetation significantly promoted the dissipation of PCBs and growth of total bacteria and the potential PCBs degraders in soils. Tall fescue and alfalfa mixed cropping was most beneficial to soil bacteria and enzyme activity. Tall fescue showed highest ability for remediation of PCBs in a poor quality soil.