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, Yangling, Shaanxi 712100, P. R. China
3. College of Environmental Science and Engineering, Nankai University, Tianjin 300071, P. R. China