Importance of substrate genesis for Mediterranean soil evolution – An example from a limestone hillslope in the Estremadura (W-Portugal)

Abstract

Many land surfaces in the Mediterranean region are characterized by undulating to steep slopes. Nonetheless, geomorphological and soil-geographical research in the past has paid only little attention to such environments and soil-geographical knowledge is therefore largely based on investigation of rather flat landscapes. By studying a limestone hillslope in western Portugal, this study aims to recover information on soil substrate layering, assess the importance of substrate formation for Mediterranean soil genesis and present an allochthonous soil evolution model.The results provide clear evidence for stratified soil profiles showing lithological discontinuities. Natural and/or anthropogenic allochthonous substrate widely covers the autochthonous substrate and therefore decouples the in situ substrate from recent pedogenesis. Therefore, substrate formation and distribution is a major key for understanding soil evolution in Mediterranean hillslope environments, whereas existing autochthonous or aeolian allochthonous soil evolution concepts (A-B-C-models) do not provide plausible explanations. Based on pedogenetic models established mainly in the mid-latitudes and the tropics, our fully substrate-genetic, allochthonous soil evolution model states that the soil-geographical pattern and the soil evolutionary stage (pedogenic ageing) is regularly, ubiquitously and systematically determined by the horizontal and vertical substrate configuration. Substrate distribution is geologically and geomorphologically based and mainly takes place via aquatic processes along depressions and valleys during a reduction in the vegetation cover. Since Mediterranean hillslope environments are fragile landscape elements, progressive pedogenic aging is repeatedly interrupted by negative (erosion) or positive (coverage) regressive processes. Consequently, pedogenesis is restricted to relatively early-stage processes including decalcification, associated substrate reddening, the accumulation of secondary carbonates in lower profile sections and the formation of Ah horizons.