1. Center for Resources, Environment and Food Security; China Agricultural Univ.; Beijing 100193 P.R. China
2. Dep. of Soil Quality; Wageningen Univ.; P.O. Box 47 6700 AA Wageningen the Netherlands
3. College of Resources and Environmental Sciences, Agricultural Univ. of Hebei; Baoding 071001 China
4. Alterra, Wageningen Univ. and Research Centre; P.O. Box 47 6700 AA Wageningen the Netherlands