Formation, characteristics and eco-environmental implications of urban soils – A review

Soils in urban areas have versatile functions, and their ecological services, especially the ability to buffer and purify pollutants, are very much needed. However, the formation and characteristics of urban soils are strongly affected by human activities, and so are their functions. In urban areas, soil horizons are often irregularly established, with anthropogenic layers and a high degree of heterogeneity owing to human disturbance and infill. Soil structure is often under degradation due to artifacts and technogenic substrata, mechanical compaction and human trampling. Physical degradation, such as compaction and destruction of structure, may reduce the ability of urban soils in infiltrating water and storing capacity, thus causing a higher runoff and pollutant load to the receiving water bodies. Soil compaction is also believed to worsen city heat island effects by reflecting more radiation, and seriously reduce the soil gas capacity and exchange. However, soil sealing fully loses its service function, further deteriorating urban ecology and environment. Enrichment of various waste materials associated with human activities, including nutrient elements, heavy metals and organic pollution, is the major problem of the urban soil environment. Materials containing rich liming in the urban soils lead to alkaline soil conditions, which can change element speciation and activities. The main features of urban soil contamination are characterized by strong accumulation of so-called “urban elements” such as copper (Cu), zinc (Zn), lead (Pb) and mercury (Hg), but less accumulation of other heavy metals. During the process of urban development, heavy metal contamination of urban soils not only happens nowadays, but also did in the past, especially when primitive mining and metal processing prevailed. Furthermore, urban soils are often polluted by organic pollutants, especially polycyclic aromatic hydrocarbons (PAHs). Platinum group elements (PGE), especially platinum (Pt), palladium (Pd) and rhodium (Rh) from automobile exhaust catalysts, are also accumulated in urban soils as affected by traffic. Physical and chemical degradation decrease soil enzymes and microbial activities, especially in industrial zones with high concentrations of heavy metals, low concentrations of organic matter and serious compaction. Unfortunately, the evolution of urban soil and attenuation of ecological function are believed to ultimately harm human welfare. Therefore, it is very important to establish a risk assessment system for better management of urban soil resources.