Assessment of native shrubs for stabilisation of a trace elements-polluted soil as the final phase of a restoration process

Abstract

Re-vegetation is the main aim of ecological restoration projects, where the use of native plants is recommended over exogenous species, which may result in an undesirable modification of the ecosystem. A 10-year phytoremediation programme was carried out in a site affected by the toxic spill of pyritic (iron sulphide) residue at Aznalcóllar (Spain) in 1998, contaminated with heavy metals (Cd, Cu, Pb and Zn) and arsenic. The success of the re-vegetation of the area with native species after a large (6 years) active phytoremediation intervention was evaluated during 4 years as the final step of the ecological restoration process. Mediterranean native shrubs (Retama sphaerocarpa, Tamarix gallica, Rosmarinus officinalis and Myrtus communis) were selected and their potential for restoration of the soils affected by the pyritic residue was assessed. Plant survival was negatively affected by soil acidity, which was the main factor controlling trace elements (TEs) solubility and soil microbial biomass, and therefore, soil quality. Nevertheless, the surviving plants were well developed and reached a large size at the end of the experiment (except M. communis). Trace element transfer from soil to harvestable parts was low for all species, and some species have been able to decrease TEs availability in the soil. The results suggest that R. sphaerocarpa was the most adequate plant species for the restoration of these soils, as it showed the highest survival rate, elevated tolerance to strong soil acidity and low TEs transfer factors.