Depth-dependent effects of vegetation restoration on soil quality in ion-adsorbed rare earth leach residues: Insights from soil functions

• Function-based assessment of soil quality after rare earth residue restoration. • Fern and graminoid vegetation restoration most effectively improves soil quality in leach residues. • Soil functions demonstrated a greater improvement in the topsoil than in the subsoil. • The dominant controls on soil quality shift depth from nutrient cycling in topsoil to nutrient provisioning in subsoil. • Carbon sequestration remains a consistent driver across soil depths. Ion-adsorption rare earth mining causes substantial soil degradation and ecosystem disruption. Vegetation restoration is a key strategy for ecological rehabilitation. However, it remains unclear how the restoration of rare earth heap leach residues under different vegetation types affects the soil quality index (SQI)—an integrative measure of multiple soil functions, and whether these effects are consistent across soil depths. Therefore, we surveyed soil profiles (0–60 cm) under three vegetation types—a fern ( Dicranopteris pedata ), a graminoid ( Miscanthus × giganteus ), and a conifer (P inus elliottii )—as well as in natural primary forest and bare land, which served as controls in ion-adsorption rare earth leach residues. The results are highlighted as follows: (1) after vegetation restoration, soil properties across the 0–60 cm profile changed substantially: physical properties by − 3.9 %–93.3 %, chemical properties by − 50.8 %–429.7 % and biological properties by − 77.0 %–2166.7 %；(2) using a function-based framework to assess SQI 0-60 , herbaceous species (SQI increased by 61 %) demonstrated superior efficacy in restoring soil quality compared to conifers (−2%); and (3) vegetation restoration effects exhibited depth-dependent dynamics, with SQI 0-20 reaching 0.36 and SQI 20-60 reaching 0.29. Revegetation led to depth-dependent improvements in soil functions, with significantly greater increases in the topsoil than in the subsoil for carbon sequestration, nutrient provisioning and nutrient cycling. Moreover, the major determinants of SQI varied with depth: carbon sequestration and nutrient cycling exerted dominant effects on SQI 0-20 , whereas carbon sequestration and nutrient provisioning were the primary contributors to SQI 20-60 . Collectively, our findings demonstrate that vegetation restoration significantly enhances topsoil quality, particularly through the use of pioneer herbaceous species, which exhibited strong functional responses during the early stages of rare earth leach residue restoration. The identified soil indicators and functions sensitive to different vegetation types provide a basis for multi-objective management strategies aimed at improving soil quality in degraded mining areas.