Weathering of soils in alpine areas as influenced by climate and parent material

Two soil sequences in northern Italy (Val di Fiemme and Val Genova) along an elevational gradient ranging from moderate (950 m a.s.l.) to high alpine (2440 m a.s.l.) climate zones were investigated with respect to element losses (Ca, Mg, K, Na, Fe, Al, Si, Mn) and development of clay minerals. Soils formed on paleo-rhyolitic parent material in Val di Fiemme and on tonalitic-granodioritic morainic material in Val Genova. All the soils have a similar age (similar to12,000 y) and have been classified as Podzols. The soils are very acid and the pH values tend to increase with decreasing altitude. Podzolization processes were most intense in the range of the subalpine forest up to the timberline (1400-1900 m above sea-level (a.s.l.)). Element leaching was greatest in this range and weathering rates decrease with both higher and lower altitudes. Due to the different lithologies and precipitations between the two valleys, the total amount of chemical weathering was slightly different, although the same trends with altitude could be observed. Imogolite-type materials (ITM) are generally of minor importance. Greater concentrations of ITM were observed in the Bhs or Bs horizons of the Episkeleti-Entic Podzols at the lower altitudes. Iron eluviation was similar in all Podzols while larger amounts of eluviated Al were detected in Val Genova. The pattern of smectite distribution along the climosequences had similarities to the trend of cation losses. The largest amount of low-charge expandable minerals seems to exist in the range of the subalpine forest up to the timberline. The development of clay minerals with a smaller layer charge was more advanced in Podzols on rhyolitic material where smectite could be detected in the Bhs and Bs horizon. Parent material influenced chemical weathering in the soils along the two climosequences and essentially determined the degree of weathering and the formation of clay minerals.