1. Centre for Ecology and Hydrology; Wallingford OX10 8BB UK
2. Dep. of Crop Soil and Environmental Sciences; Univ. of Arkansas; Fayetteville AR 72701
3. Dep. of Soil Science; Univ. of Manitoba; Winnipeg Manitoba R3T 2N2 Canada
4. USDA, Agricultural Research Service, Pasture Systems and Watershed Management Research Unit; University Park PA 16802