1. State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau; Northwest A&F University; Yangling 712100 PR China
2. Key Laboratory of Restoration and Reconstruction of Degraded Ecosystem in North-western China of Ministry of Education, United Center for Ecology Research and Bioresource Exploitation in Western China; Ningxia University; Yinchuan 750021 PR China