Soil vulnerability assessment in mountain areas: the Val d'Aosta example

Mountain ecosystems are highly vulnerable to flash flood phenomena, which can result in sudden soil water saturation and surface erosion. Saturated soils are vulnerable to solifluction and liquefaction, depending on soil plastic properties. Surface erosion and mass movements create serious hazards for the population and the infrastructures: the recognition and mapping of areas characterized by different vulnerability to erosion processes are of primary importance in order to properly manage the land use and the hydro-geological hazards. The study area is the whole Val d’Aosta region, characterized by a wide range of geological substrates, vegetation types, altitude variations and pedoclimates (from perudic in south-east and at high altitude to xeric-ustic in the intra-alpine central valley). All genetic horizons from 150 soil profiles have been described and analyzed (standard chemical analysis and Atterberg limits), in order to understand how pedogenic processes influence soil plastic properties; the results were also statistically correlated with parent material, plant cover, and pedoclimate. Liquid (LL) and plastic (PL) limits are positively correlated with humified organic matter and with pedogenic iron oxides, thus a decreasing depth trend is visible for most kinds of soils (high stability in A and Bs and low in C horizons). Both properties depend on soil pedogenic development degree, which in turn is related with pedo-climate and anthropic disturbances. The parent material is another important factor influencing soil stability: at similar development degree, carbonate and mica-rich soils show much lower liquid (LL) and plastic (PL) limits and much larger depth trends than other soils.