Enhanced phytoremediation of chlorobenzoates in rhizosphere soil

The use of plants to detoxify contaminated soil sites has the potential to be a cost-effective alternative to traditional remediation technologies. However, plant-bacteria interactions in contaminated soils are not well understood. In this study we investigated the effect of bacterial seed inoculants on the rhizosphere community during the reduction of 2-chlorobenzoic acid (2CBA) concentrations by Dahurian wild rye (Elymus dauricus). Soil was amended with 450 mg 2CBA kg−1 and aged for 2 yr, at which time the detectable amount of 2CBA was 61 mg kg−1. Dahurian wild rye inoculated with either Pseudomonas aeruginosa strain R75, P. savastanoi strain CB35 or a 1:1 mixture of these bacteria was grown in aged contaminated soil for 56 d in a growth chamber. The potential of rhizosphere soil to degrade 3-chlorobenzoic acid (3CBA), a contaminant with a similar degradation pathway to 2CBA, versus 2,3-dichlorobenzoic acid (23diCBA) or 2,5-dichlorobenzoic acid (25diCBA), contaminants with pathways dissimilar to 2CBA, was also assessed. Inoculating Dahurian wild rye with the mixed inoculum had decreased the extractable 2CBA from 61 to 29 mg kg−1, 56 d after planting but had no effect on plant growth. Inoculating Dahurian wild rye with a mixture of strains R75 and CB35 increased the potential of rhizosphere microorganisms to reduce 3CBA concentrations by 17% but had no effect on amounts of 23diCBA or 25diCBA. In a sterile hydroponic plant growth system, inoculation of Dahurian wild rye had no effect on 2CBA concentrations; although, the inoculum became established and grew in the hydroponic solution indicating that inoculants required an unknown soil factor to degrade 2CBA. Bacterial seed inoculants selectively enhanced the potential of the rhizosphere community to degrade certain compounds without affecting heterotrophic bacterial communities.