1. Dep. of Mathematical Sciences and Technology; Norwegian Univ. of Life Sciences; P.O. Box 5003 1432 Ås Norway
2. Bioforsk Soil and Environment; Fredrik A. Dahls vei 20 1432 Ås Norway
3. Dep. of Chemistry, Biotechnology and Food Sciences; Norwegian Univ. of Life Sciences; P.O. Box 5003 1432 Ås Norway
4. Stockholm Environment Institute; Kräftriket 2B 10691 Stockholm Sweden