Steel slags are major by-products of the steel and iron industry. Particularly rich in metal trace elements, these materials are massively stored in slagheaps and can constitute potential nuisance for the environment and public health. Several studies have shown the effectiveness of phytostabilization processes for rehabilitating sites contaminated by high levels of metals, both because of their sustainable and ecological aspects but also due to their low implementation costs. However, implementing a phytostabilization project for metallurgical slags is a challenging issue. Besides the potential toxicity of metal elements (e.g. Cr, Zn, Cu, V, Ni, Mo, W, Al), slags are almost devoid of organic matter and major nutrients such as nitrogen and phosphorus. They also have a very low water holding capacity (sandy texture), as well as a high pH (>10) that greatly reduces the phytoavailability of essential micronutrients (e.g. Fe, Cu, Zn). The slagheap studied located near Rive-de-Gier (France) contains approximately 500 000 tons of slags spread over an area of about 4 ha. A previous study carried out on the same site has demonstrated that the use of "Composted Sewage Sludge" (CSS) as an organic amendment favors the installation of metallophytes herbaceous species naturally able to grow on this type of substrate. Recent studies have also demonstrated the benefits of using AMF (Arbuscular Mycorrhizal Fungi) isolates for the phytostabilization of metal polluted soils and alkaline mine spoils. In this work, we evaluated the combined effects of CSS and AMF inoculation on: i) the soil pore water composition and metals bioavailability and ii) the vegetation in terms of biomass production as well as accumulation of trace metals in leaves. Results showed that AMF inoculation led to root colonization and improved P uptake. We also observed that the CSS amendment has a critical role on plant growth by changing the fluid pH (from 10.5 to 8.5) and thus, the speciation of Al in the system. Indeed, geochemical modelling run with the PhreeqC program revealed that Al becomes soluble, and therefore toxic for plant development, at a pH higher than 9. We acknowledge the financial support of School of Mines and ANR “HYPASS” supervisors.