1. Key Lab. of Arable Land Conservation (North China), Ministry of Agriculture, College of Land Science and Technology China Agricultural Univ. Beijing China
2. Dep. of Agroecology Aarhus Univ. Tjele Denmark
3. Dep. of Environmental Science The Univ. of Arizona Tucson AZ 85721 USA
4. State Key Lab. of Soil and Sustainable Agriculture Institute of Soil Sciences Chinese Academy of Sciences Nanjing China