1. State Key Laboratory of Soil Erosion and Dryland Farming on the Loess PlateauInstitute of Soil and Water Conservation, Northwest A&F University Yangling Shaanxi 712100 China
2. Institute of Soil and Water ConservationChinese Academy of Sciences and Ministry of Water Resources Yangling Shaanxi 712100 China
3. The Lanzhou Scientific Observation and Experiment Field Station of Ministry of Agriculture for Ecological System in Loess Plateau Area Lanzhou Gansu 730050 China