Assessing the impact of urban sprawl on soil resources of Nanjing city using satellite images and digital soil databases

Abstract

The Yangtse delta area is one of the most rapidly developing areas in China. There are mega-cities like Shanghai and Nanjing and the surrounding urban areas of different sizes including those along the lower reach of the Yangtse river from Shanghai to Nanjing. In combination with their satellite counties and towns, they form one of the most densely distributed urban areas in China. This is a case study conducted in Nanjing city to evaluate the impact of urban sprawl on soil resources using satellite images and digital soil database maps. The extent of the developed land in the study area and the impact of development on soil resources at a scale of 1:200,000 are estimated. The soil types occupied by the urbanization process are determined by overlaying the soil map on the satellite images (Landsat TM) of the study area at different times (1984, 1995, 2000 and 2003). This study uses a geographic information system (GIS) to combine urban land use maps of different times derived from satellite images with data on soil characteristics contained in soil databases. The results document the rapid expansion of urbanization in Nanjing city, as well as the soil types occupied by the urbanization process, and their quality. The urban area has increased 43,544 ha, 2 times more than in 1984. The urban area expanded at an annual rate of 6.9%. Thirty of the total 32 soil types (soil families) within the city were utilized by the urbanization process among which Loamy typic- Fe- leachic- stagnic anthrosol ranked the highest (12,007 ha). The loss of surface land to urban use in Nanjing city has ranged from 4.8% in 1984 to 11.8% in 2003. Soils of the first class (5349 ha) and second class (20,781 ha) were 61.5% of the total occupied soil area. Results for Nanjing show that residential, commercial, and industrial development, known as “urban sprawl,” appear to follow soil resources, with the better agricultural soils being the most affected. Several soil types appear to be on the verge of being replaced by urban sprawl. Growing urbanization may threaten food security, soil diversity and sustainability. The extent and geographic distribution of soil quality and the pedodiversity for land presently under urbanization in the study area may be determined through modeling.