1. College of Resources and Environmental Sciences; China Agricultural University; Beijing 100193 China
2. Centre for Crop Systems Analysis; Wageningen University; Wageningen 6700 AK The Netherlands
3. Department of Soil Quality; Wageningen University; Wageningen 6700 AA The Netherlands
4. Institute for Agricultural Sciences; Department of Environmental Systems Science; Swiss Federal Institute of Technology; ETH-Zurich Zurich CH 8092 Switzerland
5. Institute of Soils and Fertilizers; Gansu Academy of Agricultural Sciences; Lanzhou 730070 China