1. College of Natural Resources and Environment, Northwest A&F University, Yangling, People’s Republic of China
2. Key Laboratory of Plant Nutrition and the Agri-Environment in Northwest China (Ministry of Agriculture), Northwest A&F University, Yangling, People’s Republic of China
3. College of Surveying and Geo-informatics, North China University of Water Resources and Electric Power, Zhengzhou, People’s Republic of China
4. Department of Renewable Resources, University of Alberta, Edmonton, Canada