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

Abstract

The rhizosphere is a narrow zone of soil influenced by plant root and their exudates and its extent varies with soil type, plant species, age and many other factors. The rhizosphere microenvironment therefore has different characteristics from those of bulk soils. In the present study, the physicochemical parameters, exchangeable micronutrients (extracted with 1 M NH 4Cl), available micronutrients (extracted with Mehlich-III: Fe, Mn, Cu, and Zn), toxic metals (Cr, Pb, Ni, Cd), the Fe associated with the organic matter (extracted with Na-pyrophosphate) and the poorly crystalline Fe (extracted with 0.2 M oxalic acid) were studied in the bulk soil and in the rhizosphere of the spontaneous vegetation growing at an abandoned copper mine. The natural vegetation colonising the mine dump included two species of Ericaceae ( Erica cinerea, Calluna vulgaris) and Salix atrocinerea, whereas only the first two grew on the mine slope. The results showed that plants influence the physicochemical conditions of the rhizosphere, although not all in the same way. Thus the pH of the rhizosphere of the ericaceous species was extremely low, approximately 2–3 units lower than the pH of the bulk soil, whereas the pH of the Salix rhizosphere remained close to the respective bulk soil, which has a pH of 6.6. Furthermore, the concentration of organic matter was higher in the rhizosphere than in the bulk soil. In general the same pattern was observed for the most labile fractions of the micronutrients and toxic metals, with higher concentrations in the rhizosphere than in the bulk soil. However, there were clear differences between the ericaceous species and Salix, which prevents general conclusions being reached about the processes to which trace metals are submitted in the rhizosphere. On the other hand, although the elements under study are included in the term trace metals, they display different geochemical behaviour. Thus the exchangeable fraction was clearly correlated with the TOC for all micronutrients except Cu, presumably because the organic matter selectively adsorbs this element, thereby reducing its mobility. Finally, as regards the potential toxicity, only the concentrations of Cu and Fe surpassed the limits above which phytotoxicity may occur, while toxic metals were present at very low concentrations.