1. State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation Chinese Academy of Sciences Yangling 712100 China
2. Dep. of Plant Nutrition, College of Resources and Environmental Sciences China Agricultural University Beijing 100094 China
3. College of Resources and Environmental Sciences Qingdao Agricultural University Qingdao 266023 China
4. College of Resources and Environmental Sciences Henan Agricultural University Zhengzhou 450000 China
5. Institute of Soil Science and Fertilizer Shanxi Academy of Agricultural Sciences Taiyuan 030031 China