1. School of Natural Sciences; University of Stirling; Stirling FK9 4LA UK
2. Institute of Soil Science and Land Evaluation, University of Hohenheim; Emil-Wolff-Str. 27; Stuttgart 70593 Germany
3. Agrosphere; ICG-4, Forschungszentrum Jülich GmbH; Jülich D-52425 Germany
4. USDA-Agricultural Research Service, 3218 Williams Hall; NCSU Campus Box 7619; Raleigh, NC 27695-7619 USA