1. Center for Resources, Environment and Food Security; China Agricultural Univ.; No. 2 Yuanmingyuan Xilu, Haidian Beijing 100193 P. R. China
2. Dep. of Soil Quality; Wageningen Univ.; P.O. Box 47 6700 AA Wageningen the Netherlands
3. Alterra, Wageningen Univ.; Research Centre; P.O. Box 47 6700 AA Wageningen the Netherlands
4. Center for Animal Health and Productivity; Univ. of Pennsylvania, School of Veterinary Medicine; 382 West Street Rd. Kennett Square PA 19348