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Effect of slope aspect on transformation of clay minerals in Alpine soils

¹ Department of Geography, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland, ² Istituto Sperimentale per lo Studio e la Difesadel Suolo, PiazzaD’Azeglio 30, 50121 Firenze, Italy, ³ Museo Tridentino di Scienze Naturali, Via Calepina 14, 38100 Trento, Italy, and ⁴ ETH Zurich, Institute for Geotechnical Engineering, 8093 Zurich, Switzerland

^* E-mail: megli{at}geo.unizh.ch

(Received 2 January 2007; revised 21 June 2007)

Two soil profile sequences on paragneiss debris in the Val di Rabbi (Northern Italy) along an altitude gradient ranging from 1200 to 2400 m a.s.l. were studied to evaluate the effect of aspect on the weathering of clay minerals. All the soils had a coarse structure, a sandy texture and a low pH. Greater weathering intensities of clay-sized phyllosilicates (greater content of smectites) were observed in soils on the north-facing slope. On the south-facing slope, smectite was found only in the surface horizon of the soil profile at the highest altitude. Hot citrate treatment of north-facing soils revealed the presence of low-charged 2:1 clay minerals, the expansion of which was hindered in the untreated state by interlayered polymers. However, the hot citrate treatment encountered some problems with the samples of the south-facing soils: as confirmed by Fourier transform infrared spectroscopy, the hot citrate treatment was unable to remove all interlayer Al polymers. The 2:1 phyllosilicates were not expanded by ethylene glycol solvation in several samples, although thermogravimetric analyses indicated the presence of clay minerals with interlayer H₂O. At the same time, the collapse of clay minerals to 1.0 nm following K-saturation was evident. Theoretically, this should indicate that 2:1 phyllosilicates had no evident substitution of trioctahedral cations (Mg²⁺, Fe²⁺) by dioctahedral cations (Al³⁺ and Fe³⁺). X-ray diffraction analysis of the d₀₆₀ region and determination of the layer charge of clay minerals by the long-chain (C₁₈) alkylammonium ion, however, did not confirm this. A transformation from trioctahedral to dioctahedral species was observed and low-charge clay minerals ( ~0.30) were identified in the surface horizons of the south-facing sites. In the south-facing soils, the podzolization process was less pronounced because of a lower water flux through the soil and probably less complexing organic molecules that would remove the interlayer polymers. Besides the eluviation process, clay minerals underwent a process of ionic substitutions in the octahedral sheet that led to the reduction of the layer charge. This process was more obvious in the north-facing sites.

KEYWORDS: smectite, Na-citrate treatment, weathering, soils, microclimate, aspect