Pedogenesis in a soil catena on serpentinite in north-western Italy

Abstract

Serpentinite is a fairly common parent material for soils of the Alps and Apennines in Italy. In the Apennines these soils may show very different degrees of development even over small areas. This work was undertaken to study the pedogenesis on a soil catena occurring on a very small hill, with a particular attention to the clay mineral transformations. The catena is only 100 m long, but four orders of the Soil Taxonomy were represented, ranging from Entisols to Alfisols. In the clay fraction of the Entisol and of the Inceptisol located respectively at the summit and at the backslope positions, we found that serpentine is the main mineral in the clay fraction and that traces of some interstratified low charge vermiculites are present in the surface horizon. The two Alfisols at the footslope positions have different clay minerals: one of them shows some low charge vermiculites in the surface horizons and smectites in the deepest ones, while in the other one smectites were found in the surface horizon too. These two profiles have different drainage regimes as seen in the field and from laboratory determination of water content and porosity. Significant differences ( p < 0.01) of porosity were found in the catena between the horizons with smectites and those with low charge vermiculite. In the Mollisol, located at the toeslope position, a trend towards acidification was found in the surface horizon but the presence of serpentine acts as a continuous magnesium source thus preventing the base saturation to become < 50%. The main pedogenic processes in this catena are: lessivage of clays; formation of different 2 : 1 phyllosilicates depending probably on the porosity; acidification caused by organic materials. In some horizons an enrichment of organic matter is possible, but the acidification is hindered by the parent rock. No clear evidence of chlorite formation from serpentine was found. The weathering of serpentine minerals seems to give rise to a low charge vermiculite in upper and drier horizons or to a smectite in poorly drained conditions. Vermiculite in turn may transform into smectite under a moister environment.