ECOLOGICAL SERVICES OF URBAN SOILS IN RELATION TO URBAN ECOSYSTEM AND ENVIRONMENTAL QUALITY

Urbanization represents a dramatic change of land use or land cover. The service and function of soils undergo a fast change during urbanization, i.e., from that of sustaining plant growth to that of buffering, transforming and purifying pollutants. Urban soils thus are playing an important role in making a complete urban ecosystem and in protecting urban environment. Quality of urban soil changes with the transformation of soil functions, which include physical, chemical and biological aspects. Urban soils often get degraded due to the impact of heavy human activities. Physically speaking, soil degradation occurs mainly when soils are artificially compacted, which increases soil bulk density and lowers water infiltration rate, thus an increased surface runoff coefficient and a higher load of non-point source pollution to urban water system. Chemically, urban soils are often contaminated by organic and inorganic pollutants, which increase the accessibility of toxic substances to biological (including human) health and, meanwhile saturate the carrying capacity of soils and increase the mobility thus release risk of pollutants contained in soils. The loop of element translocation in urban ecosystem causes inevitably accumulation of nutrients and pollutants in urban soils, therefore urban soils are becoming a geochemical barrier of various elements. It is called to close the loop by a better management of urban soils in order to facilitate a healthy urban ecosystem.