Micronutrients and toxic trace metals in the bulk and rhizospheric soil of the spontaneous vegetation at an abandoned copper mine in Galicia (NW Spain).

The rhizosphere, a narrow zone of soil influenced by plant roots and their exudates, was investigated to determine the physicochemical parameters, exchangeable and available micronutrients, toxic metals, Fe associated with organic matter and poorly crystalline Fe. Natural vegetation colonising the dump included two heathers, Erica cinerea and Calluna vulgaris, and the willow Salix atrocinerea, whereas only the heathers grew on the mine slope. The results showed that plants influence the physicochemical conditions of the rhizosphere, although not all in the same way. For the ericaceous species, rhizosphere pH was approximately 2-3 units lower than the pH of the bulk soil, whereas for Salix it remained close to that of the bulk soil at 6.6. The concentration of organic matter, toxic metals and the most labile fractions of micronutrients was higher in the rhizosphere than in the bulk soil, though there were clear differences between the ericaceous species and Salix, which prevented general conclusions being reached about the processes to which trace metals are submitted in the rhizosphere.The trace elements under study displayed different geochemical behaviour, the exchangeable fraction being clearly correlated with total organic content for all micronutrients except Cu. Only the concentrations of Cu and Fe surpassed the limits above which phytotoxicity may occur, while toxic metals Cr, Pb, Ni and Cd were present only at very low concentrations.
The rhizosphere, a narrow zone of soil influenced by plant roots and their exudates, was investigated to determine the physicochemical parameters, exchangeable and available micronutrients, toxic metals, Fe associated with organic matter and poorly crystalline Fe. Natural vegetation colonising the dump included two heathers, Erica cinerea and Calluna vulgaris, and the willow Salix atrocinerea, whereas only the heathers grew on the mine slope. The results showed that plants influence the physicochemical conditions of the rhizosphere, although not all in the same way. For the ericaceous species, rhizosphere pH was approximately 2-3 units lower than the pH of the bulk soil, whereas for Salix it remained close to that of the bulk soil at 6.6. The concentration of organic matter, toxic metals and the most labile fractions of micronutrients was higher in the rhizosphere than in the bulk soil, though there were clear differences between the ericaceous species and Salix, which prevented general conclusions being reached about the processes to which trace metals are submitted in the rhizosphere.The trace elements under study displayed different geochemical behaviour, the exchangeable fraction being clearly correlated with total organic content for all micronutrients except Cu. Only the concentrations of Cu and Fe surpassed the limits above which phytotoxicity may occur, while toxic metals Cr, Pb, Ni and Cd were present only at very low concentrations.