Physico-chemical and microbial characterization in (near-) rhizosphere soil planted with macrophytes to investigate PAHs and TEs phytoremediation processes at the soil-root interface
Résumé
Rhizosphere is the key location for soil-plant transfer since it is the major interface for nutrient and water absorption by plants. Moreover, plant roots and their associated microbes play a specific role in phytoremediation: absorption of metals by plant roots and concentration into harvestable plant parts, degradation and volatilization of organic contaminants, immobilization and reduction in the mobility and bioavailability. Thereby, we compared the rhizospheres of three macrophytes, Juncus effusus, Iris pseudacorus, Phalaris arundinacae, grown two years as monospecies in representative large scale outdoor mesocosms. Mesocosms were co-contaminated with Polycyclic Aromatic Hydrocarbons (PAHs, i.e. phenanthrene, pyrene and benzo(a)pyrene), and Trace Elements (TEs, i.e. zinc, lead, and cadmium). PAHs were extracted from soils by microwave assisted extractions and analysed by gas chromatography (GC-MS) while microwave-acid digestion was used to extract metals, which were then analysed by ICP-AES. Soil and plant TEs contents and root biomass allowed us to estimate the coefficient of soil-plant transfer. The structure and diversity of the microbial communities in rhizosphere soils were also investigated by total, fungal and bacterial biomasses determination using Q-PCR, 16S rDNA and 18S rDNA analysis. Metabolic activity was evaluated measuring Fluorescein DiAcetate (FDA) and xylanase enzymatic activities. A multi-parameter approach was then used in order to illustrate the specific influence of rhizosphere in both control and spiked mesocosms. The results allowed us to determine the best macrophytes-soil system for TEs and PAHs remediation. It was correlated with the microbial development in the rhizosphere. In order to compare this approach (from mesocosm scale to field performance) and apply our results to a real case study, grass and vegetated swale receiving laterally road runoff water were also investigated for PAHs and TEs remediation.