The effects of exposure and climate on the weathering of late Pleistocene and Holocene Alpine soils

The main aim of this study was to examine the influence of exposure and consequently climate, on the chemical weathering of soils which had developed after the ice retreat of the last glaciation in Northern Italy. This was done by comparing soils developing at northand south-facing sites on siliceous parent material. There is very little data available on weathering rates and organic matter (OM) as a function of climate and exposure in such environments. Weathering rates (elemental leaching) over the whole lifetime of the soils are higher on north-facing sites. Total organic C and N contents, organic matter stocks and organic matter fractions were analysed to decipher the causes of this difference in weathering behaviour. For the organic matter fractions, we compared the easily oxidisable and stable (resistant to H2O2 treatment) organic matter fractions, water-soluble phenolic materials and alkaline-extractable fractions of the various sites. The abundance of soil organic carbon (SOC) tends to have a non-linear climate dependency. The highest amounts of SOC were measured near the timberline. In addition, compared to south-facing sites, soils on north-facing slopes have a higher organic matter content and a significantly lower degree of humification. Undecomposed or weakly degraded organic matter accumulated on north-facing sites due to less favourable thermal conditions and a higher acidity. With northern exposure, fulvic acids were more easily transported within the soil profile than humic acids and predominately gave rise to the migration (eluviation) of Fe and Al compounds due to their -COOH and -OH functional groups. Furthermore, water-soluble phenolic materials, which are more abundant on north-facing sites, have accelerated the leaching of Al. Accumulation of weakly degraded OM and the subsequently higher production of organic ligands have enhanced the eluviation of Fe and Al. Patterns of weathering processes in Alpine environments are strongly linked to biological and (micro)climatic factors which give rise to distinct differences between northand south-facing sites. The effects of exposure and climate on the weathering of 1 late Pleistocene and Holocene Alpine soils 2 3 Markus Egli*, Giacomo Sartori, Aldo Mirabella, Daniele Giaccai 4 5 Department of Geography, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, 6 Switzerland 7 Museo Tridentino di Scienze Naturali, Via Calepina 14, 38100 Trento, Italy 8 Istituto Sperimentale per lo Studio e la Difesa del Suolo, Piazza D'Azeglio 30, 50121 9 Firenze, Italy 10 11 *corresponding author: 12 tel. +41 44 635 51 14 13 fax: +41 44 635 68 48 14 e-mail: [email protected] 15 16 17