Contrasting soil dynamics in a formerly glaciated and non-glaciated Mediterranean mountain plateau (Serra da Estrela, Portugal)

Abstract

Few data are available on how soil erosion rates compare between surfaces of different ages because short-term processes often overprint the longer-term erosion signal. This study investigated the soil dynamics among two end-member sites, a formerly glaciated ('young', maximum glacial extent at 22–30 ka BP) and a non-glaciated ('old') area at the Serra da Estrela (Portugal). To disentangle soil distribution rates over different timeframes, isotopes for long-term (10Be), mid-term (δ13C) and short-term (239+240Pu) periods were applied together with principles of the percolation theory.The formerly glaciated area has soils with a lower degree of weathering and lower carbon content compared to soils of the ‘old‘, non-glaciated area. The selected isotopes and their distribution along the soil profiles revealed temporal differences in soil mixing process. It is hypothesised that the slightly higher elevation and formerly glaciated sites experienced cryoturbation effects over a longer period, while being less active or absent for the last few decades at the older, non-glaciated soils.The average long-term (millennia) soil erosion rates correspond to the expected higher rates at the younger surface and lower rates at the older surface. Once the formerly glaciated area became ice-free, soil erosion rates were high and decreased giving rise to average long-term rates of 101–140 [t km−2 yr−1] for the older surfaces and 176–248 [t km−2 yr−1] for the younger surfaces. In addition, seasonal freeze–thaw of the soils has persisted over a long period and affected the younger soils more intensively than the older soils. The current (last decades) soil redistribution rates, however, are up to one order of magnitude higher than the millennia rates and are controlled by surface angle and vegetation cover and less by soil texture. The more undulated, non-glaciated older surface had the highest short-term (decades) soil erosion rates in the range of 900–1700 [t km−2 yr−1], exhibits degrading conditions and relatively shallow soils. The younger soils, however, showed short-term (last few decades) average soil deposition rates of ∼ 230 [t km−2 yr−1]. Human impact (bush fires, grazing) is the cause for the currently strong soil degradation at the non-glaciated area.