Land Use Change And Its Effects On Soil Quality In A Typical Karst Watershed

The karst ecosystem is a very fragile one, which is susceptible to deterioration due to unreasonable human activities in karst area. The land use and land cover change play an important role in the deterioration of the karst ecosystem.Xiaojiang watershed of Yunnan Province, a typical karst watershed with medium size in scale, was chosen as basement to analyze the spatio-temporal variability of the land use and the soil quality, and the mechanisms of land use and the soil quality change. Based on the collection of historical data and sampling in the field, the author also made an ecological evaluation of land consolidation and rehabilitation, by using geostatistcs and GIS combined with mathematical model.Interpreting aerial photo taken at 1982 and mapping the land use through field investigations and survey in 2003, the land use maps of Xiaojiang watershed were obtained. Supported by GIS, and through analyzing the land use maps, the land use pattern change maps between 1982 and 2003 were obtained. The dynamic change in spatial pattern of land use and the driving force of land use change during the past two decades in Xiaojing watershed were analyzed. The results indicate: (1) The total land use transformed covers 610.12 km2, of which cultivated land increased by 268.5 km2 or 132.7%, but unused land decreased by 280.5 km2 or 54.36% during the past 20 years. (2) The land use change was very rapid in the past 20 years. The change speed of land use was 2.95%. Especially, the change speed of orchard land and cultivated land were 34.09% and 10.55% respectively. (3) The number of land use patchesincreased from 1014 to 1889, but the average area per patch decreased from 1.02 km2 to 0.55 km2. The spatial pattern of land use patches has changed from dominant large and scatter patches mosaic to dominant medium and scatter patches mosaic, and the types of dominating patch have also been transformed from unused land and forestland to cultivated land and forestland. (4) When the spatial pattern of land use was analyzed quantitatively by introducing the diversity index and fragmentation index, a clear picture of interference degree caused by human activities can be obtained. With the increased intensity of human activities, the diversity index increased by 4.14%, and the fragmentation index increased by 84.6%. Also, the isolation of the unused land and the forestland increased by 114.58%, and 69.64% respectively, but the isolation of cultivated land decreased by 45.86%. By analyzing the shift in the centers of unused land, forestland, or cultivated land, it was found that the spatial pattern of land use has changed dramatically during the past 20 years in Xiaojiang watershed. Centers of different land use types have changed towards different direction at different speeds. The distance in the shift of unused land center was farthest, which is 2.08 km, from northwest to southeast, and the distance in the center shift of forestland was 1.338 km from northeast to southwest, and the distance in the center shift of cultivated land was 0.222 km from southwest to northeast. (5) Through quantitative analysis by introducing social and economic data, it was clarified that the rapid growth of population and the economic development were the main driving forces of cultivated land change, and that the comparative economic benefit and the macro-policies such as ecological and environmental protection were important driving forces of forestland change in Xiaojiang watershed during the past 20 years. So the human driving force was direct factor of land use in the studied area.The new database of soil quality were established by laboratory analyzing of the samples collected in 2003 and by compared with data of 1982. Through application of Geostatistics combined with GIS, the spatio-temporal variability of the pH, organic matter, total nitrogen, total phosphorus, total potassium, available nitrogen, available phosphorus and available potassium in soil in Xiaojiang watershed between 1982 and 2003 were analyzed. Results show that: (1) The pH, organic matter, total nitrogen, the total phosphorus, total potassium, available nitrogen, available phosphorus and available potassium in soil in 1982 were 6.64, 33.3g/kg. 1.7g/kg. 1.7g/kg. 10.56g/kg, 114.98mg/kg, 10.68mg/kg, 66.04mg/kg, respectively; and those in 2003 were 6.74, 29g/kg, 1.6g/kg, 0.97g/kg, 11.05g/kg, 119.82mg/kg, 15.44mg/kg, 148.95mg/kg,respectively. Independent samples t-test of the tested indices of soil quality indicate that those indices have changed significantly during 20 years. (2) The parameters fitted by semivariogram models for of quality indices changed significantly in the past 20 years. (3) The spatial pattern of the soil quality indices changed significantly in the past 20 years in Xiaojiang watershed. The soil pH in 2003 was higher than that in 1982, and also the spatial pattern of the soil pH changed significantly, i, e., the soil pH increased in the east and southeast of the watershed, but decreased in the middle of the watershed. The content of the organic matter, total nitrogen and total phosphorus in 2003 was significantly lower than that in 1982, and also the spatial pattern of the soil organic matter, total nitrogen and total phosphorus changed significantly. The content of the soil organic matter decreased in the east, southeast and southwest of the watershed, but increased in the northeast and middle of the watershed. The content of the soil total nitrogen decreased in the east of watershed, but increased in the middle of watershed. The content of the soil total phosphorus decreased in the entire watershed. But the content of the total potassium, available nitrogen, available phosphorus and available potassium in soil in 2003 higher than that in 1982, and also the spatial pattern of those soil quality indices changed significantly. The content of the soil total potassium increased in the southwest and southeast of the watershed, but decreased in the middle of the watershed. The content of the soil available nitrogen increased in the middle of the watershed, but decreased in the southeast. The content of the soil available phosphorus increased in the middle and northeast of the watershed, but decreased in the southeast. The content of the soil available potassium increased in the middle, southwest and northeast of the watershed. (4) The increase of soil pH was mainly attributed to the transformation of the forestland and unused land into the cultivated land. The decrease of the content of the organic matter, total nitrogen and total phosphorus in soil was mainly attributed to the transformation of the forestland and unused land into the cultivated land, which enhanced water and soil erosion. The increase of the available nitrogen, available phosphorus and available potassium was mainly attributed to the use of more fertilizer.Through quantitative analysis by introducing relative soil quality index, it show that the soil quality decreased obviously in the past 20 years in Xiaojing watershed, but also the soil quality spatial pattern changed significantly. The total soil quality changed area covers 567.9 km2, of which the soil quality of 341.7 km2 decreased, but 226.2 km2 improved. The soil quality decreased in the southeast and southwest of the...