城市土壤的生态服务功能演变与城市生态环境保护

城市化是急剧的土地利用变化过程。在城市化过程中，土壤的服务功能经历明显的转变，即从维系植物生长的功能为主转变为滞留、转化和净化污染的功能为主，因此，城市土壤在城市生态系统中承担着重要的生态服务功能并起着保护城市环境的作用。城市土壤在功能转变的过程中，土壤质量也发生重要的转变，这些变化包括物理的、化学的和生物学的等方面。巨大的人为影响致使城市土壤的退化比较明显。物理退化方面主要体现在由于压实导致容重增加、水分入渗速率降低，从而使降雨时地表径流系数增加、非点源污染负荷升高，对城市水体污染产生明显的影响；化学退化方面主要体现在土壤中的有机和无机污染物含量显著增加，这一方面使土壤对生物健康直接影响的可能性增加，另一方面使土壤的吸附容量接近饱和丽导致污染物的移动性提高，从而具有较高的释放潜力。城市生态系统中物质循环过程不闭合的特点，决定了城市土壤是养分和污染物末端固定者，因而城市土壤是一个人为的某些元素的地球化学“垒”(barrier)。建设健康的城市生态环境需要闭合开放状态的物质循环链条，这必然需要科学的城市土壤管理策略。

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.