Microrelief and spatial heterogeneity of soils on limestone, Sub Ural plateau, Russia: Attributes and mechanism of formation

Complex soil cover and microrelief are common for arid environments. Understanding of the genesis and functioning of complex soil cover associated with microrelief is important to ensure optimal land use. We studied chalk polygons formed on the shallow limestone in the south of the European part of Russia including microrelief and soil morphology, and the main physical and chemical properties of the representative horizons. The microrelief consists of regular microhighs separated by polygonal network of elongated microdepressions. The soil cover includes Rendzic Endoleptic Calcaric Phaeozem (Relictiturbic, Tonguic) in microdepressions, Skeletic Calcaric Regosol (Relictiturbic, Raptic, Ochric) in microslopes and Skeletic Calcaric Regosol (Relictiturbic, Raptic, Nudiyermik) in microhighs. The soils consist of three main layers with abrupt boundaries: dark grey and grey, brownish-pale turbated, and white soft and hard limestone. Hard limestone from the bottom of microdepressions is connected with the surface white chalk caps on the microhighs via the soft limestone intrusions across the microslopes. Morphological analysis and micromorphology revealed a number of cryogenic features such as large wedge-shaped structures in microdepressions, mosaic profile, turbated horizons, involutions and intrusions, platy soil structure. In the absence of permafrost a number of macromorphological attributes are apparently relict, while meso-and microcryogenic features could be modern, formed during winter freezing. Limestone intrusions being relict determine the modern structure and functioning of the soil complex. They consist of disintegrated limestone gravel incorporated into soft powdery limestone, and exhibit plastic behavior when wet. Rheological characteristics confirm that the material of limestone intrusion has reduced shear resistance, and a lower range of elastic behavior and yield strength. Intrusions are the channels of water migration from the wettest microdepressions to the dry microhights. Ice lenses form in wet limestone intrusions in winter. Ice formation inside the limestone intrusions induces the frost lift and refreshment of the chalk cap. Thus, intrusions support the spatial and intra-profile heterogeneity, especially of morphology, texture and particle-size distribution. Active chalk polygons have local distribution in steppe areas with shallow limestone and concave topography, and demonstrate unique morphology, and physical and chemical attributes. The arid climate, insufficient precipitation, and annual self-restoration make grazing the optimal agricultural use of these landscapes. Otherwise relict genesis of chalk polygons and their unique attributes suggest their preservation as conservation areas.

Automated summary

The study focused on chalk polygons formed on shallow limestone in southern Russia, examining the microrelief, soil morphology, and physical and chemical properties. The complex soil cover consisted of different soil types in microdepressions, microslopes, and microhighs, with distinct boundaries between them. Cryogenic features such as wedge-shaped structures and turbated horizons were found, and limestone intrusions played a key role in the modern structure and functioning of the soil complex. Ice formation in the limestone intrusions during winter induced frost lift, supporting spatial and intra-profile heterogeneity.