Does the depletion of pentachlorophenol in root–soil interface follow a simple linear dependence on the distance to root surfaces?

To understand root–soil–microbe interactions in rhizo-depletion of xenobiotics, we conducted a glasshouse study using specially designed laminar rhizoboxes which allow intact layers of near- (1–5 mm) and far- (>5 mm) rhizosphere soil to be harvested separately from root surfaces without the removal of the root material itself. Plant (Lolium perenne L.) seedlings were grown for 90 days in a soil treated with PCP at 20 and 50 mg kg⁻¹. Changes in PCP depletion, soil microbial biomass and community structure (as indicated by phospholipid fatty acids (PLFAs) profiles) with increasing distance from the root surfaces were then assessed after harvesting. Surprisingly, depletion of PCP in the planted rhizoboxes exhibited a nonlinear dependence on the distance to root surfaces, with the most rapid loss in the 2 or 3 mm near-rhizosphere layers, contrasting to the well-known linear gradient of root exudates and mineral nutrients etc. (generally, the extent gradually decreased with increasing distance from the root surface). Soil microbial biomass carbon, however, decreased linearly as expected with increasing distance from the roots. The microbial community structures as indicated by PLFA profiles showed distance-dependent selective enrichment of competent species that may be responsible for efficient PCP depletion. The results suggest that root exudates induced modifications of microbial communities in the PCP contaminated rhizosphere and spatially modified the dominant species within these communities, resulting in the nonlinear PCP depletion pattern.