Soil multifunctionality across vegetation strata in technically restored and spontaneously colonized metalliferous mine tailings

Abstract

Metalliferous mine wastes from ore processing are among the most hazardous residues (e.g., high salinity, low nutrient and organicmatter content, poor edaphic structure, high levels of metals/metalloids). These wastes were often deposited in open-air piles (minetailings). While technical restoration commonly involves capping and afforestation, spontaneous plant colonization may occur over timewhen waste conditions are not too extreme. This natural process, which implies the direct growth of plants on mine wastes, offers a potential nature-based restoration alternative. Previous studies have shown signs of soil functionality in spontaneously colonized tailings.However, it remains unclear whether different vegetation strata promote distinct levels of soil functionality in technically restored andspontaneously colonized tailings. This work assessed the influence of shrubs and trees on soil functionality in both types of metalliferousmine tailings in a Mediterranean semiarid region of southeastern Spain, using a multifunctionality index approach.Two mine tailings were selected in the former metal mining district of La Unión-Sierra de Cartagena: one technically restored (R) andone abandoned and spontaneously colonized (C). Both sites were restored or abandoned ~40 years ago, containing the same wastetype (PbS and ZnS residues), and host plant communities of similar composition. In each tailing, two environments were studied: soilsunder shrubs and under pine trees (R-Shrubs vs. R-Trees; C-Shrubs vs. C-Trees). Seasonal sampling was conducted over one year,and multiple parameters related to soil functionality were measured to compute multifunctionality indices. Four variable categories wereestablished: (i) soil processes (CO2 emission, C and N mineralization); (ii) organic matter decomposition (C, N and P cycle enzymes,dehydrogenase, phenol oxidase, peroxidase); (iii) nutrient cycling (dissolved organic C, phenols, hexoses, NH4+, NO3-, PO43-), and(iv) others (electrical conductivity, contamination degree, microbial biomass).Results showed soil functional capacity in both tailing types, with vegetation effects varying within each site. In the restored tailing,shrubs and trees exhibited similar multifunctionality index values, with no seasonal variation (R-Shrubs ~9.9-11.3, R-Trees ~11.1-12.7).Indices were largely driven by C and N cycle enzyme activity and balanced C and N mineralization. In the colonized tailing, tree soilsdisplayed higher multifunctionality index values than those under shrubs, also without seasonal shifts (C-Shrubs ~3.9-6.4, C-Trees~8.2-9.9). In this case, P cycle and oxidative enzymes (phenol oxidase, peroxidase), along with N mineralization, had the greatestinfluence on index values.