1. Gansu Academy of Agricultural Sciences, Lanzhou 730070, Gansu, P.R.China
2. State Key Laboratory of Soil Erosion and Dryland Agriculture on the Loess Plateau, Institute of Soil and Water Conservation, the Chinese Academy of Sciences, Yangling 712100, P.R. China
3. Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, P.R. China
4. Pingliang Prefecture Institute of Agricultural Sciences, Pingliang 744000, Gansu, P.R. China