[en] Organic pollutants such as polycyclic aromatic hydrocarbons (PAH) can reach high concentrations in soils due to man-made pollution related to industrial, agricultural or urban activities. Such concentrations can reach toxic values and create major environmental and health problems. One of the first entry point of pollutants in plant ecosystems is the rhizosphere, defined as the soil under the influence of roots. In the rhizosphere, the plant release root exudates, feeding soil microorganisms, and take up water and nutrients. Among the rhizosphere inhabitants, arbuscular mycorrhizal (AM) fungi are ubiquitous root symbiotic fungi, contributing to plant growth and plant nutrition. In PAH-polluted soils, biodegradation of PAH increases, which is attributed to increased microbial activity in the rhizosphere..We studied the contribution of the rhizosphere of mycorrhizal and non-mycorrhizal plants to the biodegradation of PAH in the rhizosphere, taking into account microbial community structure. Different experiments were performed with industrial contaminated soils and PAH-spiked soils, in pot cultures as well as compartmented devices allowing to analyze rhizosphere processes in consecutive sections as a function of distance to roots. Clover and ryegrass, inoculated or not with the arbuscular mycorrhizal fungus Glomus mosseae were used.. After different time periods, plants were harvested, biomass and mycorrhizal root colonization were estimated. Microbial Density of microbial heterotrophs and of degrading bacteria was estimated by MPN techniques in micro-plates. Microbial community structure was estimated by DNA extraction from the rhizosphere, amplification by PCR and analysed by TGGE (temperature gradient gel electrophoresis), or by PLFA (phospholipid fatty acid analysis). PAH in soil were extracted by Soxhlet and analysed by GC-MS. We showed that the concentration of PAH increased with the distance to roots (Corgie et al, 2003) and was lower in the myco-rhizosphere than in the rhizosphere of non-mycorrhizal plants (Joner et al, 2001; Joner and Leyval, 2003). Such processes also differed with time and the nature of the PAH. Microbial density and bacterial community structure, estimated by PCR-TGGE after DNA extraction or by PLFA, also differed as a function of distance to roots (Corgie et al, 2004), with time and with the presence/absence of mycorrhizal fungi (Joner et al., 2003). Such results show that there are clear gradients of microbial community and microbial activity in the rhizosphere, that could explain gradients of biodegradation. They also show that in such experiments priming effects can mask rhizosphere effects on PAH degradation (Joner et al, 2004). Possible implications for the phyto-remediation of PAH-polluted soils will be discussed. (authors)
; (no.10); p. 195