1. College of Natural Resources and Environment, Northwest A&F University , Yangling 712100 , China
2. UWA Institute of Agriculture and UWA School of Agriculture and Environment, The University of Western Australia M062 , 35 Stirling Highway, Crawley, WA 6009 , Australia
3. The State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, CAS&MWR , Yangling , China
4. Department of Renewable Resources, University of Alberta , Edmonton, AB T6G 2H1 , Canada
5. Key Laboratory of Plant Nutrition and the Agri-Environment in Northwest China (Ministry of Agriculture), Northwest A&F University , Yangling 712100 , China