Weathering rates as a function of climate: results from a climosequence of the Val Genova (Trentino, Italian Alps)

A soil sequence in northern Italy along an elevational gradient ranging from moderate (950 in a.s.l.) to high alpine (2440 m a.s.l.) climate zones was investigated with respect to element losses (Ca, Mg, K, Na, Fe, Al, Si, Mn) and the development of clay minerals. Element losses were derived from mass balance calculations that are based on enrichment/depletion factors using immobile element contents (Ti). Mass balance calculations indicated that extensive mineral weathering resulted in significant leaching losses of major base cations (with losses up to >80% of the initial content in the parent material), Al (particularly, from upper horizons) and Si. Element denudation was greatest in subalpine forests near the timberline. Weathering rates decreased with both higher and lower altitudes. The pattern of smectite distribution along the climosequence has strong similarities to the Al, Si, Na and K losses, while an inverse trend could be detected for clay minerals with a high amount of interlayered Al. Highest smectite contents with up to 30% of the clay fraction and also highest weathering rates were found at sites in subalpine forest near the timberline. Higher precipitation rates and the production of chelating compounds in the soil are believed to promote the appearance of smectites. Weathering rates of Na and K were additionally modelled as a function of precipitation and temperature. The relationship between climate and element weathering is strongly nonlinear and is thought to be overshadowed by the pronounced podzolisation effect near the timberline. (C) 2002 Elsevier Science B.V. All rights reserved.