Quality of Constructed Technogenic Soils in Urban Gardens Located on a Reclaimed Clay Pit

Urban gardening plays diverse social, cultural and economic roles; its further development appears to be worthwhile, provided that soil contamination does not compromise ecosystem services. This study was conducted at a complex of urban gardens in Wroclaw (Poland) where topsoil screening indicated significant spatial differentiation of trace elements content, presumably related to the history of the site. Urbic Technosols cover the reclaimed section of the gardens, where industrial and urban waste materials, such as ash, slag, construction and demolition, and household waste, were used to fill former clay and sand mines. Although the topsoil layers, comprised of transported external soil, exhibited beneficial physicochemical properties and high fertility, they were seriously contaminated with trace elements (up to 1700, 920, 740, 5.1, 7.4, and 5.1 mg kg−1 zinc, lead, copper, cadmium, mercury, and arsenic, respectively). The trace elements were likely transferred from technogenic materials used for mine infilling, which now underlie the thin humus layers of the garden soils. The results suggest that the quality of soils in urban gardens located at reclaimed post-mining sites, while seemingly beneficial for horticulture based on physicochemical soil properties and fertility indices, can be seriously and permanently compromised by soil contamination from inappropriate materials used for site reclamation, thereby affecting soil quality and posing potential health and ecological risks.