1. College of Resources and Environmental Sciences, Agricultural Univ. of Hebei; Baoding 071001 P. R. China
2. Center for Resources, Environment and Food Security; China Agricultural Univ.; Beijing 100193 P. R. China
3. Dep. of Soil Quality; Wageningen Univ. and Research Center; P.O. Box 47 6700 AA Wageningen the Netherlands
4. College of Agriculture and Natural Resources, Univ. of Delaware; 116 Townsend Hall Newark DE 19716