Soil function assessment in smelting sites: Conceptual evolution, methodological advances, and future directions for...

Soil functions can be understood as bundles of processes arising from the interactions among soil physical, chemical, and biological properties. The extent to which soils deliver these functions is largely shaped by land use. Smelting sites, as a distinctive land-use type, present challenges such as the absence of standardized assessment frameworks, trade-offs among soil functions, and uncertainties in site-specific applications. This review synthesizes soil function's conceptual evolution (contrasting with soil quality frameworks) and proposes a standardized, staged assessment framework for smelting site soils-initial, remediation, and utilization stages-based on heavy metal contamination levels. For each phase, the corresponding functions, proxy indicators, and methodological approaches are summarized. In the initial stage, the buffering-filtration-retention potential for heavy metals is emphasized, with core indicators including total and bioavailable metals, pH, cation exchange capacity (CEC), and field capacity. In the remediation stage, biological vitality and nutrient cycling are prioritized, with indicators such as available nitrogen (N), phosphorus (P), potassium (K), organic matter (OM), and microbial biomass. In the site utilization stage, natural recovery capacity is highlighted, with indicators encompassing buffering-filtration capacity, soil water content, permeability, and biological activity. Among various assessment approaches, the total comprehensive score method is identified as the most feasible and robust for evaluating soil multifunctionality in smelting sites. By bridging knowledge gaps and advancing a systematic evaluation scheme, this review provides a conceptual and methodological foundation for soil function restoration, sustainable land use, and the protection of human and ecosystem health in post-industrial landscapes.