Formation of placic horizons in soils of a temperate climate – The interplay of lithology and pedogenesis (Stołowe Mts, SW Poland)

A placic horizon is a thin soil layer that is cemented or indurated by Fe, Mn, and/or Al compounds as well as by organic matter. The placic horizon is a hard, continuous, and nearly impermeable and impenetrable horizon that retards the vertical leaching of water and inhibits the growth of roots. Placic horizons can develop under different climates and in various ways. However, we do not fully understand the interactions between lithology and pedogenesis that might promote placic horizon formation. Therefore, to shed light on the mechanisms of placic genesis, we used an applied multiproxy approach (electrical resistivity tomography – ERT, XRD, ⁵⁷Fe Mössbauer spectroscopy, bulk geochemistry, soil micromorphology and ¹⁴C dating) for three soils developed from sandstone–mudstone parent materials in mountainous areas of SW Poland, to shed light on the mechanisms of placic horizon genesis. The ERT inversion models and soil survey data indicate lithic discontinuities in the profiles. Soil micromorphology data confirmed that a placic horizons formed slightly below the discontinuity. Radiocarbon ages of the placic horizon span from 2.2 to 4.8 ka and suggest that more humid conditions were favorable for the formation of a placic horizon, which aligns with our mineralogical results. Ferrihydrite is the major Fe oxyhydroxide in the placic horizon. Micromorphological data showed that after the formation of the placic horizon, a phase of clay migration might have occurred, while later, podzolisation took over as the main soil-forming process. This sequence shows that thin iron pans can develop independently of other processes and can exist (at least some of them) before the development of a spodic horizon. A placic horizon causes severe hydrological changes in the topsoil and subsurface horizons. Thus, water stagnation in the soil transforms it into a Stagnosol or Planosol and it’s responsible for the evolution of local plant communities and the ecosystem.