Plant root exudates impact the hydrocarbon degradation potential of a weathered-hydrocarbon contaminated soil

Phytoremediation is a promising low cost technology for the cleanup of contaminated sites. However, specific plants may promote degradation under one set of conditions but not under another, and knowledge limitations surrounding the mechanisms of phytoremediation hamper attempts at optimization. We addressed this issue by examining how exudates released by Elymus angustus (wildrye) and Medicago sativa (alfalfa), grown under hydrocarbon-stressed or non-stressed conditions, impacted the degradation potential of microbial communities in a weathered hydrocarbon-contaminated soil. Degradation potential was assessed using mineralization assays with 14C-labeled hydrocarbons (hexadecane, naphthalene, phenanthrene) followed by DGGE of microbial communities and quantitative-PCR of genes associated with hydrocarbon degradation. All root exudates repressed hydrocarbon mineralization in soil microcosms, with exudates from hydrocarbon-stressed wildrye having the least repressive impact. Changes in degradation potential were not associated with changes in the dominant microbial community structure or with significant shifts in general microbial abundance. Degradation was, however, associated with functional changes in microbial communities. Mineralization of polyaromatic hydrocarbons (PAH) was highly correlated with copy numbers of catechol 2,3 dioxygenase and naphthalene dioxygenase, two genes involved in PAH degradation. Both gene copy numbers and mineralization parameters were significantly impacted by exudate composition, with specific compounds associated with either increased (acetate, alanine) or decreased (malonate) degradative capacity. The success of a given phytoremediation treatment is likely influenced by the relative amount of these and similar compounds within root exudates.

► Mechanisms of phytoremediation rhizosphere effect still unclear.
► We examine the influence of plant exudates on hydrocarbon degradation.
► Exudates impacted function but not overall structure of soil microbial communities.
► Specific exudates associated with changes in soil microbial degradative capacity.