Karst has been regarded as a fragile environment by environmental scientists. Because of karst system with a low capacity, it is difficult to restore if once disturbed. Changes in environment are not unique throughout the karst region, but a karst region is more sensitive than other regions. In the present study we analyzed the land use changes from 1982 to 2003, and assessed the effects of land use changes on pH value, organic matter (OM), total nitrogen (TN), total phosphorus (TP), total potassium (TK), available nitrogen (AN), available phosphorus (AP), available potassium (AK) in soils of Xiaojing watershed, a typical karst agricultural region in Yunan province, Southwest China. The results indicate: (1) The total land converted covers an area of 610.12 km2, of which 134.29 km2 of forestland were converted into cultivated land, and 210 km2 of unused land were converted into cultivated land during the past 20 years in Xiaojiang watershed. (2) The rapid growth of population and the economic development were the main driving forces of cultivated land change, and the comparative economic benefits from the macro-policies such as eco-environmental protection were important driving forces of forestland change in Xiaojiang watershed during the past 20 years. (3) Because of land use change, the soil properties have been changed significantly. The pH value, OM, TN, TP, TK, AN, AP and AK in soils in 1982 were 6.3, 38.02 g/kg, 1.86 g/kg, 1.63 g/kg, 10.94 g/kg, 114.42 mg/kg, 11.65 mg/kg and 64.69 mg/kg, respectively; and those in 2003 were 6.73, 25.26 g/kg, 1.41g/kg, 0.99 g/kg, 12.6 g/kg, 113.43 mg/kg and 11.11 mg/kg, 151.59 mg/kg, respectively. Pared samples t-test of the tested indices of soil properties indicate that those indices have changed significantly during the past 20 years. (4) Because of the differences of land use change, the soil properties change differently. The contents of the soil OM, TN, TP in 2003 were significantly lower than those in 1982 after the forestland and unused land were converted into cultivated land, but the pH of soil increased significantly in 2003, especially the properties of the soils developed from the carbonate rock strata changed most significantly. The soil properties change was mainly attributed to deforestation, water and soil erosion, and the low land management level after the forestland and unused land were converted into the cultivated land. Because of lack of the vegetation cover and the land management, the contents of soil OM, TN, and TP for short-time reforested land also decreased, but the soil pH increased. Because of the use of more fertilizer and the improvement of land management, the soil properties improved significantly after the cultivated land was converted into orchard land. But due to unreasonable human activities to the fragile soils in the karst region, the land became rock desertified. The content of the soil OM, TN, TP, AN and AP decreased significantly, but the soil pH increased significantly after land converted became rock desertified. (5) Also, with the changes in land use and soil ameliorative measures, the modifications of properties of soils developed from the carbonate rock strata were more sensitive than those from the sandstone strata.
