Cley minerals, oxyhydroxide formation, element leaching and humus development in volcanic soil.

Aweathering sequence with soils developing on volcanic, trachy-basaltic parent materials with ages ranging from 100–115,000 years in the Etnaregion served as the basis to analyse and calculate the accumulation and stabilisation mechanisms of soil organic matter (SOM), the transformationof pedogenic Fe and Al, the formation and transformation of clay minerals, the weathering indices and, by means of mass-balance calculations, netlosses of the main elements. Although the soils were influenced by ash depositions during their development and the soil on the oldest lava flowdeveloped to a great extent under a different climate, leaching of elements and mineral formation and transformation could still be measured.Leaching of major base cations coupled with a corresponding passive enrichment of Al or Fe was a main weathering mechanism and was especiallypronounced in the early stages of soil formation due to mineral or glass weathering.With time, the weathering indexes (such as the (K+Ca)/Ti ratio)tend to an asymptotic value: chemical and mineralogical changes between 15,000 and 115,000 years in the A and B horizons were small. In contrastto this, the accumulation of newly formed ITM (imogolite type materials) and ferrihydrite showed a rather linear behaviour with time. Weatheringconsisted of the dissolution of primary minerals such as plagioclase, pyroxenes or olivine, the breakdown of volcanic glass and the formation ofsecondary minerals such as ITM and ferrihydrite. The main mineral transformations were volcanic glass ➔ imogolite ➔ kaolinite (clay fraction). Inthe most weathered horizons a very small amount of 2:1 clay minerals could be found that were probably liberated from the inner part of volcanicglass debris. The rate of formation and transformation of 2:1 clay minerals in the investigated soils was very low; no major changes could beobserved even after 115,000 years of soil evolution. This can be explained by the addition of ash and the too low precipitation rates. In general, soilerosion played a subordinate role, except possibly for the oldest soils (115,000 years). The youngest soils with an age b2000 years had the highestaccumulation rate of organic C (about 3.0 g C/m2/year). After about 15,000 years, the accumulation rate of organic C in the soils tended to zero. Soilorganic carbon reached an asymptotic value with abundances close to 20 kg/m2 after about 20,000 years. In general, the preservation andstabilisation of SOM were due to poorly crystalline Al- and Fe-phases (pyrophosphate-extractable), kaolinite and the clay content. These parameterscorrelated well with the organic C. Imogolite-type material did not contribute significantly to the stabilisation of soil organic matter.© 2007 Elsevier B.V. All rights reserved.Keywords: Soil formation; Kaolinite; Weathering indexes; Soil organic matter; Mediterranean region; Volcanic soils