1. College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi Province 712100, P.R. China
2. Key Laboratory of Plant Nutrition and the Agri-environment in Northwest China, Ministry of Agriculture, No.3 Taicheng Road, Yangling, Shaanxi 712100, P.R. China
3. Key Laboratory of Pollution Processes and Environmental Criteria, Ministry of Education, Tianjin Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300071, P.R. China