1. Department of Plant Ecology; Justus-Liebig-University Giessen, Heinrich-Buff Ring 26-32; D-35392 Giessen Germany
2. Department for Soil Science and Plant Nutrition; Hochschule Geisenheim University; Von-Lade-Str. 1 D-65366 Geisenheim Germany
3. Isotope Bioscience Laboratory - ISOFYS; Gent University; Coupure Links 653 Ghent 9000 Belgium
4. Department of Environmental Systems Science, Swiss Federal Institute of Technology, ETH Zurich; Tannenstrasse 1 8044 Zurich Switzerland
5. School of Biology and Environmental Science; University College Dublin; Belfield Dublin 4 Ireland