1 Plant-Soil-Interactions, Agroscope, Institute for Sustainability Sciences, 8046 Zurich, Switzerland. Department of Evolutionary Biology and Environmental Studies, University of Zurich, 8057 Zurich, Switzerland. Plant-Microbe Interactions, Institute of Environmental Biology, Faculty of Science, Utrecht University, 3584 CH Utrecht, Netherlands. marcel.vanderheijden@agroscope.admin.ch.