Vertic properties and gilgai-related subsurface features in soils of south-western Poland

• 650 subsurface microlows similar to such forms under gilgai relief were found under temperate climate.
• Vertic horizons have developed in a clayey subsoil under the cover of loess (silt loam).
• Wedge-shaped peds are ordered under variable angles, and have grooved surfaces and slickensides.
• The soils have mollic and argic horizons above vertic one, and strong stagnic properties in the subsoil.
• Soils are Chernozems above microlows and Planosols or Phaeozems above microhighs.

More than 650 subsurface concave structures (1–20 m long, 0.5–7 m wide, and 0.1–0.9 m deep) were described in soils developed of clays overlain by loess in a large-scale archeological excavation in SW Poland. The structures are considered to be microlows and microhighs similar to those in gilgaied vertisols, which have never been reported in Central Europe until now. Soils do not have a clayey texture throughout the profile; however, the thick and humus-rich mollic horizons, as well as a clay-enriched argic layers, developed directly above vertic horizons have favored soil cracking and shrink–swell phenomena in the clayey subsoil. As a result, well developed wedge-shaped (lenticular) aggregates have formed in the clayey subsoil. Aggregates have grooved surfaces with an oriented matrix (slickensides) and are ordered under variable angles from 10 to 60°. A periglacial origin of these structures has been rejected and the mid- to late-Holocene period (since the Neolithic epoch) has been considered to be favorable for intense shrink–swell phenomena, vertic horizon development and clayey mass cycling responsible for the subsurface bowl and chimney structure development. A micro-mosaic of soil units related to these structures has been distinguished, including (non- or deep-vertic) Stagnic Chernozems/Phaeozems above the microlow centers, Mollic Planosols (Vertic) above the microslopes, and Mollic Planosols (Protovertic) over the microhighs (chimneys). It is concluded that a temperate climate (as in Central Europe) allows the formation of vertic properties and subsurface mass reorganization similar to those in soils with gilgai microrelief, if specific conditions – clayey substratum, position in landscape, soil moisture regime and the land use connected with vegetation type – support deep seasonal changes of soil moisture and shrink–swell phenomena in the subsoil.