An integrated strategy for recovering soil contaminated with potentially toxic elements through biochar and spontaneous vegetation

: Abandoned mining sites often act as ecological hotspots, where soil concentrations of potentially toxic elements (PTEs) exceed safe thresholds, posing ongoing risks to ecosystems and causing persistent environmental degradation. Assisted phytoremediation, by combining plants and soil amendments, constitutes a sustainable option for restoring contaminated sites. Accordingly, this study assessed whether the application of biochar (BC; 3% w/w) could promote the establishment of a natural grassland cover in a PTE-contaminated mine soil (i.e. Sb 412 mg kg-1; Cd 51 mg kg-1; Pb 2664 mg kg-1; Zn 7510 mg kg-1). Two further treatments were applied: NPK fertilizer was added to both control-soil and biochar-amended soil. Biochar addition increased soil pH (from 7.5 to 8.1), total organic C (from 1 to 6%) and CEC (from 9.6 to 12.7 cmol(+) kg-1). Labile Cd and Zn decreased by 44 and 40% respectively, versus the control, while the residual fraction of PTEs increased (e.g., Cd +16%, As +76%, Sb +11%). These changes were accompanied by an improved plant cover establishment, particularly for Lotus cytisoides L., whose growth and PTE uptake were assessed after a 28-month period. This species exhibited a strong adaptability across the treatments, reaching 100% relative frequency and up to 59% abundance in soils amended with BC and supplemented with NPK fertilizer. Translocation and bioaccumulation factors for all PTEs indicated predominant root retention in plants grown on BC-amended soils, highlighting phytostabilization as the primary remediation mechanism. Overall, BC-assisted phytoremediation improved soil safety, fertility and plant diversity, underscoring its efficacy for the ecological restoration of abandoned mining sites.