1. Department of Plant Nutrition, China Agricultural University, Beijing, 100094, P. R. China
2. Institute for Plant Nutrition Soil Science, Christian Albrecht University of Kiel, 24118 Kiel, Germany
3. Department of Agronomy, China Agricultural University, Beijing, 100094, P. R. China
4. Institute of Plant Nutrition, University of Hohenheim, 70593 Stuttgart, Germany