1. Department of Soil and Environment; Swedish University of Agricultural Sciences; P O Box 7014 SE-75007 Uppsala Sweden
2. Agroscope, Climate/Air Pollution Group; Reckenholzstrasse 191 CH-8046 Zürich Switzerland
3. The James Hutton Institute; Craigiebuckler Aberdeen AB15 8QH UK
4. Department of Ecology; Swedish University of Agricultural Sciences; P O Box 7044 SE-75007 Uppsala Sweden