1. Dep. of Soils; Federal Univ. of Santa Maria; 97105-900 Santa Maria RS Brazil
2. Dep. of Agricultural Sciences; Regional Integrated Univ. of High Uruguay and Missions (URI); 98400-000 Frederico Westphalen RS Brazil
3. EPAGRI; P.O. Box 116 89620-000 Campos Novos SC Brazil
4. INRA-AgroParisTech; UMR 1402, Ecosys, RD10 Versailles F-78026 Cedex France
5. Dep. of Environmental Sciences; Univ. of California; Riverside CA 92521