Steinmergelkeuper forest soils in Luxembourg: Properties and pedogenesis of soils with an abrupt textural contrast

Abstract

The process of clay dispersion and soil formation is investigated for an 'in situ' soil under semi-natural deciduous forest in Luxembourg on Steinmergelkeuper marls. We studied the genesis of these soils with a characteristic abrupt textural contrast between the topsoil and the subsurface soil. During and after rainfall, subsurface flow is generated, laterally transporting and exporting dispersed clay downslope at the interface between the silty topsoil and the clayey Bg horizon. This process resulted in a relative coarse silty and shallow topsoil, directly on a clayey Bg horizon. On hillslope positions, the clay content increased from 23% clay in the topsoil to 50% in the subsurface horizons. This abrupt textural contrast was less pronounced in soils on water divides and the colluvial valley bottom. The non-calcareous soils had an AEh-EAh-Bg-Bw-C horizon sequence, a near neutral pH, a very high base saturation (>96%) dominated by Ca and Mg, and a relative organic matter rich topsoil. They are classified as Luvic Planosols. The main mechanism involved in the development of the textural contrast in the soil was the swelling and dispersion of (fine) clay from the top of the Bg horizon, and its lateral transport in macropores downslope over the almost impervious Bg horizon. The high macroporosity and hydraulic conductivity finds its origin in bioturbation, and swelling and shrinking of the topsoil. The lateral transported (fine) clay was dispersed from the top of the Bg horizon, as indicated by the similarity in the clay mineralogy of clay of the Bg horizon and dispersed clay sampled in subsurface flow and stream. The absence of stabilizing agents in the Bg horizon, such as organic matter, carbonates and pedogenic sesquioxides, allowed slaking of macro-aggregates, followed by the swelling of micro-aggregated clay. This swelling was caused due to the very low electrolyte content of the soil solution, which was always below the flocculation value of the clay of the Bg horizon. In addition, the dispersion domain of this soil material was enlarged by soluble humic substances, which were adsorbed at the clay particles. This lateral eluviation process of clays, or subsurface erosion of the Bg horizon, was the main process explaining the sharp abrupt textural contrast in these soils.