1. Dep. of Agroecology, Faculty of Science and Technology; Aarhus Univ.; Blichers Allé 20, P.O. Box 50 DK-8830 Tjele Denmark
2. Dep. of Biotechnology, Chemistry and Environmental Engineering; Aalborg Univ.; Sohngaardsholmsvej 57 DK-9000 Aalborg Denmark
3. Dep. of Soil, Water and Environmental Science; Univ. of Arizona; 1177 E. Fourth Street Tucson AZ 85721
4. School of Chemical, Biological, and Environmental Engineering, Oregon State Univ.; Corvallis OR 97331