Aluminium geochemistry in the bulk and rhizospheric soil of the species colonising an abandoned copper mine in Galicia (NW Spain)

Purpose Aluminium partitioning in the solid fraction and aluminium in solution in the bulk and rhizospheric soil of different plant species colonising an abandoned Cu mine slope (Calluna vulgaris, Erica cinerea and Salix atrocinerea) and mine dump (C. vulgaris and E. cinerea) were investigated. The aim of the study was to determine the changes that the species induce in the Al forms in the rhizosphere in order to adapt to heterogeneous substrates. Materials and methods Al was extracted from the solid phase with different solutions: ammonium oxalate (Alo), sodium pyrophosphate (Alp), copper chloride (Alcu), lanthanum chloride (Alla) and ammonium chloride (AlNH4). The following Al fractions were obtained: inorganic non-crystalline Al (Alop=Alo-Alp), highly stable organoaluminium complexes (Alpcu=Alp-Alcu), organoaluminium complexes of intermediate stability (Alcula=Alcu-Alla) and labile organoaluminium complexes (Alla). The concentration of Al present in the aqueous phase was also determined. Results and discussion The pH of the soil in the mine slope was close to 7, and the roots of Ericaceae caused strong acidification so that the pH of the rhizospheric soil was low (3.6-4.7). In contrast, the pH of the bulk and the rhizospheric soil of S. atrocinerea remained close to 7. In the mine dump (pH3.7), the changes in the pH of the Ericaceae rhizosphere in relation to the bulk soil were not as marked as in the mine slope. Alop predominated in the solid phase (more than 70% of the Alo), and Alpcu predominated in the organoaluminium complexes (more than 55%), followed by Alcula (13% and 47%) and Alla (3% and 21%). The concentration of Al in solution was significantly related to the concentrations of AlNH4 (r=0.43), Alla (r=0.50) and Alcula (r=0.45). Conclusions Ericaceae species grew in dump and slope materials because they modified the pH of the rhizospheric soil, while S. atrocinerea only grew in areas where the soil conditions were close to neutral. The concentration of aluminium fractions was higher in the Ericaceae rhizosphere soil than in Ericaceae bulk soil, S. atrocinerea rhizosphere and bulk soils. Moreover, highly stable organoaluminium complexes predominated, and the dissolved Al concentration was low, despite the strong acidity.