Urban soil properties distinguished by parent material, land use, time since urbanization, and pre-urban geomorphology

The typically high lateral and vertical variation of urban soil properties challenges the interpretation and prediction of their spatial patterns. The aim of this study was to investigate physico-chemical properties of, and pedogenesis in urban soils with different parent materials, and to evaluate whether information on land use, time since urbanization, and pre-urban geomorphology can help to better understand and spatially predict urban soil properties in the historic city of Ghent, Belgium. Sixteen soils were collected from sites with various land uses in three quarters that were urbanized at different times. They were described and characterized by laboratory methods (soil pH, electrical conductivity, soil organic carbon content, black carbon content, equivalent calcium carbonate content, soil particle size distribution, coarse fragments content) and portable X-ray fluorescence (total Cr, Ni, Cu, Zn, and Pb content). Site history and pre-urban geomorphology were deducted from archeological reports and historical imagery. Our results revealed that human-transported materials (HTMs) are common in soils in Ghent. Decalcification, eluvation or illuvation in HTM were only evidenced in soils for which landfill occurred >100 years ago, which had not been disturbed recently. Hence, soil properties were strongly influenced by compositional differences (e.g., nature or type of artifacts) between (native or human-transported) parent material types. In contrast to land use, time since urbanization and pre-urban geomorphology were indicative of parent material type, soil properties, and human soil modifications. More specifically, soils in the old, medieval quarter Inner City consisted mainly of urbic or spolic parent material and had a lower sand content, and a higher content of silt, coarse fragments, and heavy metals (Ni, Cu, Zn, Pb) compared to the young quarter Wondelgem, where soils mainly developed in native parent material or lithic HTM. Furthermore, soils situated in raised, former floodplains had a higher soil pH, electrical conductivity, calcium carbonate content, silt content, and coarse fragments content, and a lower sand content compared to soils of the periglacial plain. Multidisciplinary data sources (e.g., historical maps) can be integrated to better comprehend the complex urban soilscape, where past and current human interventions, pre-urban geomorphology and hydrography are all at play.