Copper Biogeochemistry in Response to Rhizosphere Soil Processes Under Four Native Plant Species Growing Spontaneously in an Abandoned Mine Site in NE Brazil

We evaluated the biogeochemical processes occurring in the rhizosphere of different native plants growing spontaneously in a copper-contaminated soil in an abandoned mine site in NE Brazil. We also assessed the effects that these processes have on copper mobility and toxicity and discuss the potential use of the plants as pioneer species in restoration programs. For these purposes, we determined chemical (pH, macronutrients, % TOC, and % TIC) and mineralogical (XRD) properties in both rhizosphere and nonrhizosphere soils (bulk soil), and we used the sequential extraction method (SEM) to extract copper from both soils. The study findings show that the plants have greatly altered the physicochemical characteristics of the soil that is directly influenced by their roots. Different plant species appear to act through different processes, thus altering various soil components and affecting the biogeodynamic cycling of essential nutrients and copper. The changes in the physical-chemical characteristics of the rhizosphere affected copper dynamics, mainly manifested as significantly lower concentrations of potentially bioavailable copper, i.e., exchangeable and carbonate-associated copper, in this soil fraction. The concentration of copper associated with noncrystalline Fe oxides was also higher in the rhizosphere, thus enhancing the immobilization and probably minimizing the risks of copper toxicity and mobility. The biogeochemical processes observed in the rhizosphere of the species under study seem to indicate that the plants promote phytostabilization of copper in their rhizosphere zone, and they thus show desirable characteristics for use in phytoremediation programs.