The association of the legume Anthyllis vulneraria and the grass Festuca arvernensis, was found to be very efficient for the phytostabilisation of highly multi-metal contaminated mine tailings. Our objective was to quantify the contribution of Anthyllis inoculated with its symbiotic bacteria Mesorhizobium metallidurans to the soil N pool and to test whether a starter nitrogen fertilization may improve symbiotic nitrogen fixation and the growth of Festuca. Methods Plants of Festuca and of Anthyllis inoculated with M. metallidurans were grown separately during eight months in pots filled with mine contaminated soil. Estimation of the N fluxes was realized using 15?N isotopic methods. Results Starter N fertilization (28 kg N ha?1) improved symbiotic N2 fixation and the growth of both species. Belowground N balance (N rhizodeposition - soil N uptake) of the non-fertilized Anthyllis at maturity was negative (?30.6 kg N ha?1). However, the amount of N derived from fixation, including above- and belowground parts, was 78.6 kg N ha?1, demonstrating the ability of this symbiotic association to improve soil N content after senescence. Conclusions i) soil N enrichment by the N2-fixing symbiotic association occurs after plant senescence, when decaying leaves and shoots are incorporated into the soil; ii) application of a starter fertilization is an efficient solution to improve phytostabilisation of highly contaminated sites.