Weathering and evolution of soils formed on granitic, glacial deposits: results from chronosequences of Swiss alpine environments

Abstract

Two soil chronosequences of mountainous ecosystems in Switzerland served as the basis to calculate the accumulation of soil organic matter, transformation of pedogenic Fe and Al and net losses of the main elements (Ca, Mg, K, Na, Fe, Al, Mn and Si) by means of mass-balance calculations. Elemental losses due to deglaciation and exposure to the weathering environment were calculated. These mass balance calculations indicate that extensive mineral weathering resulted in significant leaching losses of Si, major base cations, and Al (particularly from the upper horizons). The losses are especially pronounced in the early stages of soil formation. In most cases, the exponential decay model incorporating an asymptotic or logarithmic regression model seems to provide a good description of weathering. The greatest changes in the soil chemistry of these alpine soils on granitic host material occurred within the first 3000–4000 years of soil development. Later, the weathering rates decreased rapidly and the overall depletion of elements nearly reached an asymptote. We also found that the mean ratio of [Al tFe d]/[Al dFe t] of the fine earth of the A, E and B horizons is closely linked to the duration of soil development. A very rapid decrease of this ratio also occurred at the beginning of soil evolution in order to reach asymptotic values after about 3000 years. Consequently, this ratio could be a good indicator of the age of alpine soils. Furthermore, there is a close relationship between the mass of organic C or N in the whole profile and the soil age: the older the soil the higher the corresponding mass per unit area. The chronofunctions presented give a first attempt of the chemical soil evolution in the Alps. However, only little data has been available up until now regarding alpine soils, and this fact inhibits a more detailed observation of the changes that have occurred over a period of 2000–8000 years of soil formation under similar climatic conditions.