1. College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi Province 712100, PR China
2. Key Laboratory of Plant Nutrition and the Agri-environment in Northwest China, Ministry of Agriculture, Yangling, Shaanxi 712100, PR China
3. Institute of Soil and Water Conservation, Northwest A&F University, Yangling, Shaanxi Province 712100, PR China