Rhizosphere bioremediation of polychlorinated biphenyls (PCBs) offers a potentially inexpensive
approach to remediating contaminated soils that is particularly attractive in remote regions including
the Arctic. We assessed the abilities of two tree species native to Alaska, Salix alaxensis (felt-leaf willow)
and Picea glauca (white spruce), to promote microbial biodegradation of PCBs via the release of phytochemicals
upon fine root death. Crushed fine roots, biphenyl (PCB analogue) or salicylate (willow secondary
compound) were added to microcosms containing soil spiked with PCBs and resultant PCB
disappearance, soil toxicity and microbial community changes were examined. After 180 d, soil treated
with willow root crushates showed a significantly greater PCB loss than untreated soils for some PCB
congeners, including the toxic congeners, PCB 77, 105 and 169, and showed a similar PCB loss pattern
(in both extent of degradation and congeners degraded) to biphenyl-treated microcosms. Neither P. glauca
(white spruce) roots nor salicylate enhanced PCB loss, indicating that biostimulation is plant species
specific and was not mediated by salicylate. Soil toxicity assessed using the Microtox bioassay indicated
that the willow treatment resulted in a less toxic soil environment. Molecular microbial community analyses
indicated that biphenyl and salicylate promoted shifts in microbial community structure and composition
that differed distinctly from each other and from the crushed root treatments. The biphenyl
utilizing bacterium, Cupriavidus spp. was isolated from the soil. The findings suggest that S. alaxensis
may be an effective plant for rhizoremediation by altering microbial community structure, enhancing
the loss of some PCB congeners and reducing the toxicity of the soil environment.