Long-Term Biogeochemical Stabilization of Carbonate-Bearing Pb–Zn–Cu Mine Tailings by Amendment-Assisted Phytostabilization

Abstract Legacy Pb–Zn–Cu mine tailings can remain barren and geochemically reactive for decades. Although amendment-assisted revegetation can establish short-term plant growth, sustained phytostabilization requires metal attenuation and soil-like development over years after amendment-derived nutrients and organic matter decline. We evaluated seven-year biogeochemical stabilization of carbonate-bearing tailings planted with the native legume Dalea purpurea. Biosolids (BS), biochar (BC), and arbuscular mycorrhizal fungi (AMF) were incorporated into the upper 8 cm, while the underlying 12 cm remained unamended. Tailings pore water was monitored annually, and year-seven cores were analyzed by depth for organic carbon, PLFA-based microbial indicators, aqua-regia-extractable metals, sequential metal fractionation, and X-ray diffraction. Biosolids initially increased EC, DOC, DN, and dissolved Pb, Zn, and Cu, whereas BC- and/or AMF-containing treatments showed greater long-term reductions in dissolved metal concentrations. AMF-inoculated treatments increased organic carbon and microbial biomass indicators below the amended layer, while deeper root penetration was also observed, consistent with profile-scale biological development. Sequential extraction showed metal-specific repartitioning: Pb was primarily carbonate- and Fe/Mn oxide-associated; Zn remained partly carbonate-linked; and Cu shifted toward organic/sulfide-associated and residual pools, consistent with lower relative lability. These findings show that surface-applied amendment and planting strategies can promote coupled biological and geochemical stabilization in initially unamended deeper tailings layers, supporting ecosystem restoration beyond short-term surface revegetation.