1. Key Lab. of Plant-Soil Interactions, Ministry of Education, Key Lab. of Arable Land Conservation (North China), Ministry of Agriculture; College of Resources and Environment, China Agricultural Univ.; Beijing 100193 PR China
2. Dep. of Grassland Science; China Agricultural Univ.; Beijing 100193 PR China
3. Dep. of Agroecology, Faculty of Science and Technology; Aarhus Univ.; Blichers Allé 20 8830 Tjele Denmark