Classification of Urban Soils

Permanent technogenic disturbances of urban environments and formation of technogenic sediments result in short cycles of soils’ formation and ‘young’ age of soils of urban areas. Moreover, different susceptibility of urban soil materials to anthropogenic disturbances result in different ages of urban soils’ horizons. Dust sedimentation and greenery maintenance contribute to the vertical growth of soil layers. This trend of ‘topsoil’ buildup is referred as ‘synlithogenic’ trend in soil forming process. Synlithogenic soil formation is typical for urban soils and, in contrast, is rare for natural soils, where the major soil processes usually are directed down in the profile (except, for alluvial, colluvial and volcanic soils) (Dobrovolsky and Urussevskaya, 2004). In result, the relative age of urban topsoil is most often younger than of subsoil layers. Various types of soil transformations in urban areas can be distinguished: (i) transport and deposition; (ii) long-term deposition; (iii) mixing; (iv) sealing (Hulisz et al., 2016). The most typical features of soil formation in described areas include: i) vertical growth of topsoil layers and predominantly synlithogenic soil formation process; ii) short time periods for soil formation, resulting in the early stages of pedogenesis; iii) abrupt and clear boundaries of layers and horizons, iv) specific chemical features, caused by dust deposition and anthropogenic disturbances, including alkaline pH, contamination with heavy metals and hydrocarbons, elevated carbon and phosphorous content; v) altered physical features, including high bulk density and high share of technogenic materials (artefacts) within the profile; vi) specific community of living organisms both in terms of biodiversity and total biomass. Urban soils are then frequently characterized by a substantial horizontal and vertical heterogeneity. The differentiation of the soil sites is significantly related to the site age, technogenic impact intensity and the form of land use (Greinert, 2015). Specific factors of formation of urban soils and their unique features determines substantial differences between technogenic and non-urban soils, recognized by many national and international classifications, which distinguish presently urban soils as an individual taxon. However, modern soil science, derived from agronomy and forestry, since its beginning in late 19th century was focused on natural zonal and azonal soils, whereas soils of the urban areas were absent in soil classification schemes for a long time. Present-day status of these soils in the international and selected national classifications will be presented in this chapter