Biogeochemistry of high altitude soils on ophiolites in the Western Italian Alps.

Soils on ophiolitic materials are usually rich in Mg, Fe and heavy metals, with a pH value close to neutrality and a high base status. The low Ca/Mg ratio and the high metal content can cause toxicity symptoms on biological communities. Thus, plant associations are often peculiar and rich in adapted species. Chemical and biological properties of alpine and arctic soils formed on these materials have seldom been studied worldwide. 97 soil profiles (associated with phytosociological surveys) were opened and analyzed above the treeline in Mont Avic Natural Park (Val d’Aosta, north-western Italy), 27 of which in heavily cryoturbated or snowbed habitats (above 2550 m above sea level). The aim was to verify the active pedogenic processes, the mobility and bioavailability of heavy metals, to recognize the edaphic properties influencing plant distribution and possible toxic effects on microbial and arthropodal communities. The climate is intra-alpine (average temperatures between 2 and -3°C, average rainfall below 1300 mm/y); water scarcity is never a limiting factor for vegetation. The substrate is composed of ophiolitic rocks, with serpentinite as the most common rock type and minor gabbros, amphibolites and calcschists. Soils are usually acidic on all substrates, with pH values increasing from stable, low altitude sites to heavily cryoturbated and eroded ones. Substrate influences the particle size distribution (fine on serpentinite), the Ca/Mg ratio (often lower on ultramafic materials) and heavy metal content. Pedo-geomorphic processes (cryoturbation and freeze-thaw cycles-induced erosion and deposition) “rejuvenate” soils by adding non-weathered material on the top of the profile: this phenomena differentiate the edaphic properties on the different substrates and increase the metal concentration in biologically active horizons on serpentinite. The low pH values and the waterlogging at snowmelt cause the release of the metals from the primary minerals, thus increasing the mobile and bioavailable contents: for example, in heavily cryoturbated soils on serpentinite (Turbic Cryosols or Turbic Regosols), the EDTA-extractable Ni reaches 1000 mg kg-1. The presence of high available metals in deep horizons of downslope soils, formed on metal-poor materials, show the existence of water flowing at the interface between soils and impermeable or frozen deep layers, carrying large quantities of potentially toxic metals in solution. Plant communities are well correlated with edaphic properties, the most important of which is bioavailable Ni. The statistical methods show that the typical species occurring on acidic soils (for example, Carex curvula and Luzula spicata) are absent despite the low pH values, because of high Ni concentration. They are replaced by endemic species, well adapted to metals (Thlaspi sylvium, T. rotundifolium subsp corymbosum and Carex fimbriata). Despite the great impact on vegetation, bioavailable metals don’t have a strong influence on microbial and microarthropodal communities. Microbial activity (biomass and respiration) and stress indices (metabolic quotient and TOC/biomass) are not related either with substrate or metal content.