1. JordforskCentre for Soil and Environmental Research, N-1432 Ås, Norway, University of Oslo, Department of Biology, P.O. Box 1050 Blindern, N-0316 Oslo, Norway, Risø National Laboratory, PBK 313, P.O. Box 49, DK-4000 Roskilde, Denmark, and National Environmental Research Institute, Vejlsøvej 25, DK-8600 Silkeborg, Denmark