Dynamics of PAHs and derived organic compounds in a soil-plant mesocosm spiked with 13C-phenanthrene.

Polycyclic Aromatic Hydrocarbons (PAHs) are ubiquitous and persistent soil pollutants. Their fate and the influence of the plant rhizosphere on their dynamics has been extensively studied, but studies mainly focused on their dissipation rate. We conducted a plant-soil mesocosm experiment to study the fate and distribution of PAHs or derived compounds in the extractable fraction, the residual soil, the shoot biomass and the root biomass. The experiment was conducted for 21 days using ryegrass and a forest soil spiked with 13C-labeled phenanthrene (PHE), using combined IRMS and NanoSIMS for analyses. Almost 90% of the initial extractable PHE content was dissipated within 3 weeks, but no rhizospheric effect was highlighted on PHE dissipation. More than 40% of 13C-PHE was still in the soil at the end of the experiment, but not as PHE or PAH-derived compounds. Therefore it was under the form of new compounds (metabolites) and/or had been incorporated into the microbial biomass. About 0.36% of the initial 13C-PHE was recovered in the root and shoot tissues, representing similar 13C enrichment (E13C) as in the soil (E13C ≈ 0.04 at.%). Using NanoSIMS, 13C was also localized at the microscale in the roots and their close environment. Global 13C enrichment confirmed the results obtained by IRMS. Some hotspots of 13C enrichment were found, with a high 32S/12C14N ratio. Comparing the ratios, sizes and shapes of these hotspots suggested that they could be bacteria.