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. College of Biology and the Environment, Nanjing Forestry University, Nanjing 210037, China
4. College of Environmental Science and Engineering, Nankai University, Tianjin 300385, China
5. Department of Civil & Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong 00000, PR China