Soil, regolith, and weathered rock: Theoretical concepts and evolution in old-growth temperate forests, Central Europe

Abstract

Evolution of weathering profiles (WP) is critical for landscape evolution, soil formation, biogeochemical cycles, and critical zone hydrology and ecology. Weathering profiles often include soil or solum (O, A, E, and B horizons), non-soil regolith (including soil C horizons, saprolite), and weathered rock. Development of these is a function of weathering at the bedrock weathering front to produce weathered rock; weathering at the boundary between regolith and weathered rock to produce saprolite, and pedogenesis to convert non-soil regolith to soil. Relative thicknesses of soil (Ts), non-soil regolith (Tr) and weathered rock (Tw) can provide insight into the relative rates of these processes at some sites with negligible surface removals or deposition. Scenarios of weathering profile development based on these are developed in current study. We investigated these with ground penetrating radar, electrical resistance tomography, and seismic profiling at three old growth forest sites in the Czech Republic, on gneiss, granite, and flysch bedrock.We found that the geophysical methods – which generated thousands of separate measurements of Ts, Tr, Tw – to produce good estimates. The weathered rock layer (sensu lato) was generally the thickest of the weathering profile layers. Mean soil thicknesses were about 0.64–0.75 m at the three sites, with typical maxima around 1.5 m. Non-soil regolith thicknesses averaged about 2.5 m on the gneiss site and 1.2–1.4 at the other sites. Weathered rock had a mean thickness of 7 m at the gneiss site (up to 10.3), 4.6 at the granite site, and 3.4 on flysch. Results indicate that weathering at the bedrock weathering front is more rapid than conversion of weathered rock to regolith, which is in turn more rapid than saprolite-to-soil conversion by pedogenesis on all three bedrock types. No evidence was found of steady-state soil, non-soil regolith, or weathered rock thicknesses or evolution toward steady-state. Steady-state would require that weathering rates at the bedrock and/or regolith weathering fronts decline to negligible rates as profiles thicken, but the relative thicknesses at our study sites do not indicate this is the case.