Vegetation type modulates spatial-temporal variations in soil conditions in technically restored and spontaneously colonized metalliferous mine tailings

Mine tailings are open-air waste deposits enriched in metal(loid)s derived from ore processing. Their restoration may proceed through capping and afforestation, which isolate the tailings from the outset, or through spontaneous plant colonization, a slower process whose study provides a model for evaluating the feasibility of direct plant establishment on these materials. Although both pathways can improve soil quality and reduce metal(loid) mobility, the role of vegetation type in modulating these processes remains insufficiently understood. We assessed the effects of shrubs and trees on the physico-chemical, chemical and biological properties of upper soil layers in two metalliferous tailings in semiarid southeastern Spain: one technically restored and one naturally colonized for several decades. Over one year, we monitored soil microclimate, nutrient pools, total and water-soluble metal(loid)s, microbial attributes and ecotoxicity under shrub and tree patches. Tree soils were consistently cooler and wetter during the warm season, with temperature differences of 3–7 °C and moisture differences of 1–4 %. In the colonized tailing, trees markedly improved soil structure and biological functioning relative to shrubs, with higher aggregate stability (50 vs. 34 %), total organic carbon (12 vs. 5 g kg −1 ), microbial respiration (up to 0.9 g m −2 h −1 CO 2 ) and dehydrogenase activity (0.8 vs. 0.4 μg INTF kg −1 h −1 ). Despite very high total metal(loid) concentrations (Pb 2150–5400 mg kg −1 ; Zn 1700–3300 mg kg −1 ), water-soluble fractions remained comparatively low (100–900 μg kg −1 ) and increased mainly when dissolved organic carbon peaked under trees. • Trees promoted organic-layer formation, driving key soil changes across tailings. • Soil under shrubs showed lower biological function in colonized tailing. • Trees outperformed shrubs in stabilizing soil temperature and moisture. • Colonized and technically restored tailings showed similar tree-driven benefits. • Spontaneous tree colonization supports passive, nature-based mine tailing restoration.