Evidence supporting the value of spontaneous vegetation for phytomanagement of soil ecosystem functions in abandoned metal(loid) mine tailings

Abstract

This work studies a set of soil indicators (physical, physico-chemical and biological), evaluated under field and laboratory conditions, in different stages of spontaneous vegetation colonization in abandoned metal(loid) mine tailings from Mediterranean semiarid areas. The results provide evidence about the interest of spontaneous colonization by native vegetation for the phytomanagement of abandoned metal(loid) mine tailings in terms of providing ecosystem functions. Bare soils (B), small groups of pine trees (P), scattered pine trees with shrubs and herbs under the canopy (P+S), and dense patches of pine trees with shrubs and herbs under the canopy (DP+S) were studied inside mine tailings abandoned ≈40 years ago. Besides, pine forests next (FN) and away (FA) from the tailings were also studied. Pioneer and nurse plants were mainly found inside the tailings, although ecological indexes in P+S and DP+S were similar to FN and FA. Pedogenesis evidences such as structure development and increase in cation exchange capacity, organic C and N were found in tailing soils from B to DP+S. However, soil metal(loid)s did not follow the same variation pattern. For example (in mg kg-1): P showed the maximum total Cu (≈277) and Zn (≈17,860), while P+S of As (≈1250) and Pb (≈14,570). B had the maximum water extractable Pb (≈4) and Zn (≈207), while FA of As (≈0.192) and Cu (≈0.149). Soil microbial biomass C, enzyme activity, CO2 emission, organic matter decomposition and feeding activity of soil dwelling organisms indicated similar, or even higher, biological activity in P+S and DP+S than in FN and FA. In fact, FA showed the highest soil ecotoxicity risk (reduced enchytraeid reproduction). Therefore, mine tailing soils can be effectively modified following spontaneous vegetation colonization, achieving conditions with capacity to provide certain ecosystem functions. Hence, phytomanagement of these tailings should be preceded by a detailed knowledge of the existing spontaneously colonized sites, which should be preserved to take advantage of their potentiality.