2.3. Soil sampling and analysis
Soil samples were collected at 79 sites in Quzhou County in spring 1980 and again at the same sites in spring 1999. At each site, 3–5 (1 kg) surface soil samples at 0–20 cm depth were collected within a 100 m radius, and bulked for analysis. The distribution of soil nutrient sample sites was based on the SNSS map (1:50,000) (Fig. 1). Overlay analysis was conducted based on the SNSS map and the current land use map in 1980 and 1999 to determine the changes in land use types at the soil sample sites. The numbers of soil samples in different land use types are as follows: 22 from rainfed land, 48 from irrigated land, and 9 from virgin grassland in 1980 and 5 were from rainfed land, 18 from vegetable land and 56 from irrigated land in 1999.