1. Forschungszentrum Jülich GmbH, Institute of Chemistry and Dynamics of the Geosphere IV: Agrosphere, 52425 Jülich, Germany, Alterra Green World Research, Wageningen University and Research Centre, P.O. Box 47, 6700 AA Wageningen, The Netherlands, and Fraunhofer-Institute for Molecular Biology and Applied Ecology, P.O. Box 1260, 57377 Schmallenberg, Germany