Heavy metal fractionation and pedogenesis in subalpine and alpine soils on ophiolitic materials, western Alps

Soils on ultramafic materials are usually rich in Mg, Fe and heavy metals (particularly Ni, Cr, Mn, Co). These chemical properties could cause toxicity effects on biological communities. Metal fractionation shows the soil phases to which metals are linked (exchangeable, associated with organic matter, with amorphous or crystalline Fe and Mn oxides, residual), and thus it is strictly related with metal bioavailability. Ni, Cr, Mn, Co and Fe fractionation (6 fractions) have been analysed in 6 subalpine and in 17 alpine (i.e., above the present-day treeline) soils in the ophiolitic area of Mont Avic Natural Park (Valle d’Aosta, Italian Alps), on soils formed from metal-rich serpentinite or from metal-poor mafic rocks and calcschists. The results show a tight relationship between vegetation, soil forming processes, metal speciation and bioavailability: below and above the present-day treeline soil forming processes and metal speciation change dramatically. Under subalpine forest, the dominant pedogenic process is podzolization. In these extremely acidic and leached soils, all metals are mobilized and their lowest concentration is in the bleached E horizon, while there is a higher value in organic matter-rich surface horizon and in the illuvial B. All the fractions of Ni, Co and Mn are strictly correlated with each other: in A and Bs horizons, the greatest fraction (not in the primary minerals) is associated with organic matter and with crystalline Fe-oxides. Cr is less released by weathering, and the greatest fraction is together with organic matter and amorphous Fe-oxides. No Cr could be detected associated with Mn oxides. Above the treeline, the situation changes dramatically. Leaching is important only in stable, flat areas, while usually metal concentration increase in surface horizon because of organic stabilization. The most important factors are erosion and cryoturbation.