Aluminum speciation in the bulk and rhizospheric soil solution of the species colonizing an abandoned copper mine in Galicia (NW Spain)

Purpose The present study was carried out to identify and quantify the aluminum species present in the bulk and rhizospheric soil solution of the spontaneous vegetation colonizing the dump (Calluna vulgaris, Erica cinerea) and slope (C. vulgaris, E. cinerea, Salix atrocinerea) of an abandoned Cu mine in Touro (Galicia, NW Spain). Materials and methods Total dissolved aluminum was speciated into reactive Al (Alr) and acid-soluble Al (Alsa). Acid-soluble aluminum comprises colloids, polymers and/or organo-aluminum complexes, whereas Alr comprises non-labile monomeric aluminum (AlnL) and labile monomeric aluminum (AlL). The latter fraction included Al³⁺, Al-OH complexes, Al-F complexes, and Al-SO₄ complexes. Results and discussion The results show that Ericaceae modified the pH of the rhizosphere from 6.6 to 4.0 (C. vulgaris) in the slope, whereas S. atrocinerea only grew in close-to-neutral soil conditions and increased the rhizosphere pH by 0.8 units. The total concentration of Al in the soil solution was low (<75 μmol L⁻¹) and was higher in the rhizosphere than in the bulk soil solution in all samples. Under Ericaceae, the Alr represented between 80% and 98% of the total Al in solution and the concentration was higher in the rhizosphere; under S. atrocinerea the Alsa was always higher than 80%, with higher concentrations in the rhizosphere. AlL predominated over AlnL in all samples. The distribution of labile Al into different species depended on the pH and the concentration of labile Al: Al³⁺ predominated in all samples from the dump (>40% of the AlL) and in the C. vulgaris rhizosphere in the slope (>52% of the AlL); the Al-F complexes were more abundant in the E. cinerea rhizosphere in the slope (>40% of the AlL), and Al-OH complexes predominated in all samples associated with S. atrocinerea (>70%). In the mine dump, the Ca/AlL ratio changed from 0.07 (bulk soil) to higher than 1.4 in the root zone of the Ericaceae thus decreasing the risk of toxicity by this element. Conclusions Labile aluminum is the prevailing fraction in all dump samples and in the rhizosphere of Ericaceae growing on the slope. Al³⁺ is the predominant species in the most acidic samples with the highest concentration of labile Al and the highest AlL/F ratio. In areas where there may be a risk of Al toxicity, Ericaceae accumulated Ca in the vicinity of the roots, thus increasing the Ca/AlL ratio relative to that in the bulk soil and preventing absorption of Al and the associated phytotoxicity.