The ecological functions of microorganisms (e.g. mineralization of organic matter, nitrogen cycle, soil aggregation, etc…) modify soil properties and elements availability. They play a crucial role in nutrient uptake, making microorganisms important drivers for plant development and ecological successions. Many soil parameters can drive the adaptability or acclimation of soil living organisms like plants or bacterias. Micro-organisms adaptation is a fast process according to their short life cycle and their ability to host plasmids which can be transferred to other community members. As a drastic selective factor, high metallic soil content may promote strong and site specific micro-organisms evolution. In addition, a few studies suggested that bacteria may decrease the accumulation of toxic trace metals by plants, thereby improving plant development on highly contaminated soils. This is highly valuable for phytostabilization approaches, whose goal is to stimulate the development of a self-sustainable plant cover on polluted sites, in order to limit pollutant release and transfer to the environment.

However, to our knowledge, no specific bacterial strains are currently commercially available for phytostabilization purposes. In this respect, we performed additional studies on a phytostabilization field experiment onto metallurgical slags, to: i) document the mid-term natural response of microbial populations to the various forcing conditions and ii) identifying slag-compatible Plant Growth Promoting Bacteria (PGPB) which could help plants to grow in this extreme environment.

The study site is a metallurgical dump heavily contaminated with various metals (e.g. Cr, Zn, Cu, V, Ni, Mo, W, Al) near Rive-de-Gier, France. We took advantage of the phytostabilization experiments performed in-situ since 2010 within the ANR-PHYSAFIMM project. These field experiments were aimed at testing various amendments (ramial chipped wood, composted sewage sludge and N/P/K mineral fertilization) and plant assemblages ("regular", "metallophyte", no seeding) onto a slag substrate. Eight years after, the experimental plots show contrasting plant covers, suggesting that different communities of microorganisms could be present from one tested condition to another. Metals tolerance and other functional traits (e.g. phosphorus solubilizators, organic acid, siderophorus and/or IAA producers) were assessed on strains isolated from rhizospheric soils belonging to three plant species grown on three differently amended phytostabilization parcels.

We acknowledge the financial support of PHYTOSUDOE interreg project, School of Mines and ANR “HYPASS” supervisors. Thanks to Leticia Bravo Gomez and Maria Ángeles de Jesús Gonzalez for the help provided with strains collections.