What can carbon and nitrogen mineralization reveal about soil functionality in technically restored and spontaneously colonized metalliferous mine tailings?

Metalliferous mine tailings are inhospitable environments for biota. While technical restoration commonly involves capping and revegetation, the direct growth of plants on mine waste (e.g., spontaneous colonization) may offer a nature-based alternative when waste conditions are not overly extreme and tailings are located within natural areas. However, soil functioning and microbial behavior in spontaneously colonized tailings remain less understood compared to technically restored ones. This study evaluated the capacity of soils from technically restored and spontaneously colonized tailings in a Mediterranean semiarid region to mineralize C and N, considering vegetation type (shrubs, trees) and seasonal variability. Soils were sampled seasonally for one year and analyzed for microbial parameters, including C (glucose-induced respiration) and N (lucerne grass decomposition) mineralization. Despite high total metal(loid) concentrations (e.g., As ∼70-500, Pb ∼5400-7,200, Zn ∼3300-5800 mg kg-1), both tailing types supported microbial activity, but with differing intensities: total C-CO2 released (restored tailing ∼35-50 g kg-1; colonized tailing ∼9-16 g kg-1); N mineralization rate (restored tailing ∼0.5-1.3 mg N kg-1 d-1; colonized tailing ∼0.7-1.5 mg N kg-1 d-1). Technical restoration improved the soil microbial environment, enhancing C mineralization across both vegetation types. Additionally, N mineralization appeared to be more strongly influenced by plant-driven microclimatic conditions (e.g., shading or increased sunlight exposure) than by inherent soil properties. In contrast, in the colonized tailing, trees played a key role in promoting both C and N mineralization. C-CO2 emission dynamics indicated r-strategist dominance in restored tailing soils (continuous respiration) and k-strategist traits in colonized tailing soils (stepwise respiration).