1. Inst. for Sustainability Sciences, Agroscope; Reckenholzstrasse 191 CH-8046 Zürich Switzerland
2. Inst. of Evolutionary Biology and Environmental Studies, Univ. of Zürich; Winterthurestrasse 190 CH-8057 Zürich Switzerland
3. Dept of Microbiology; Swedish Univ. of Agricultural Sciences; SE-750 07 Uppsala Sweden
4. Inst. of Environmental Biology, Faculty of Science, Utrecht Univ.; NL-3508 TC, Utrecht the Netherlands