Non-vegetated soils are characterized by harsher conditions and thus presented lower values of biological parameters than vegetated soils. Similarly, Epelde et al. (2008a) reported lower rates of microbial biomass and activity in bare mine soils when compared with vegetated mine soils.
Importantly, vegetated and non-vegetated sites were clearly separated in the PCA. Vegetated sites were also clearly separated between themselves, reflecting a spatial variability in terms of soil pH, metal availability, plant biomass, and microbial properties, characteristic of mine tailings (Cobb et al., 2000; Córdova et al., 2011; Ernst, 2005). This site heterogeneity explains the frequently variable response of each site to the same treatment (i.e., the lack of clear treatment-induced responses in many parameters), representing one of the main problems to quantify the remediation effectiveness of such mine tailings.