1. Dep. of Agroecology Faculty of Science and Technology; Aarhus Univ.; Blichers Allé 20 P.O. Box 50 DK-8830 Tjele Denmark
2. INRA; UMR 1091 Environment and Arable Crops; INRA-AgroParisTech; F-78850 Thiverval-Grignon France
3. Dep. of Biotechnology Chemistry and Environmental Engineering; Aalborg Univ.; Sohngaardsholmsvej 57 DK-9000 Aalborg Denmark