1. Dep. of Soil and Water Sciences; China Agricultural Univ.; Beijing China 100193
2. The James Hutton Institute, Invergowrie; Dundee DD2 5DA UK
3. Division of Civil Engineering; Univ. of Dundee; Dundee DD1 4HN UK
4. Univ. Institute of Technology, Univ. of Poitiers; Poitiers 86000 France
5. Rothamsted Research, West Common, Harpenden; St Albans AL5 2JQ UK