Rhizospheric degradation of phenanthrene is a function of proximity to roots

Rhizodegradation of recalcitrant organic pollutants, such as polycyclic aromatic hydrocarbons (PAH), may benefit from the major role that root exudates have on rhizospheric microbial processes. We investigated the influence of the proximity to ryegrass ( Lolium perenne L.) roots on microbial populations and their biodegradation of phenanthrene (PHE) using compartmented pots. PAH degrading bacteria, total heterotrophic bacteria and PHE biodegradation were quantified in three consecutive sections at different distance ( 0 - 3, 3 - 6, 6 - 9 mm) from a mat of actively exuding roots. A bacterial gradient was observed with higher numbers of heterotrophs and PAH degrading bacteria closest to the roots. In parallel, a PHE biodegradation gradient was evident in the presence of roots. The biodegradation reached 86%, 48% and 36% of initially added PHE, respectively, in the layers 0 - 3 mm, 3 - 6 and 6 - 9 mm from the roots. The biodegradation rate was similar throughout the three layers of the non planted control. The present experimental system seems well suited for spatial and dynamic studies of PAH rhizoremediation.