Pedogenesis in disturbed alpine soils (NW Italy).

In alpine environments, natural or man-induced disturbances are fairly common and acknowledged as determining factors in pedogenesis and soil distribution. We have selected a representative alpine valley in the Italian Northwestern Alps to evaluate the effects of perturbations of different intensity and frequency on soil development and weathering during the last few centuries. In order of decreasing disturbances, we selected: (i) an active avalanche shoot; (ii) a man-built terrace; (iii) a park-like forest; and (iv) a coniferous forest. In the two most disturbed sites, independently of the intensity and frequency of the disturbances, the active pedogenic processes are restricted to the topsoil. At site 1, the acidity induced by the invading pioneer N2-fixing tree species determines a certain degree of mineral dissolution, while at site 2, plowing and manuring influence the properties of organic matter and limit its mobility. At site 3, where coniferous species are associated to the herbaceous cover, the weathering proceeds further and a cambic horizon was detected in the field and supported by evidences of K depletion from illite. The higher polarity of the organic substances of mixed origin is certainly responsible for this more intense weathering. In the coniferous forest, the least disturbed situation, the vegetation is more representative of the climax in the area and pedogenesis further proceeds leading to the appearance of podzol-like features. These, even if scarcely displayed by a very shallow and discontinuous E horizon, are clearly manifested by the formation of interstratified minerals and by the amount of pedogenic iron oxides. This can be related to the organic matter dynamics, with a high FA/HA ratio and accumulation in the Bw horizons of acidic and highly polar aromatic molecules. However, podzols are not common at this elevation and this steady state condition seems to be very vulnerable to disturbances even of low intensity. In conclusion, in an alpine valley, the impact of different disturbances on soil development, independently from their intensity and frequency, leads to a pattern of development of soils in which the vegetation plays a key role