1. Department of Biology, Universiteit Antwerpen, B-2160 Antwerpen, Belgium.
2. Max Planck Institute for Biogeochemistry, 07701 Jena, Germany.
3. Laboratoire des Sciences du Climat et de l'Environnement, 91191 Gif sur Yvette, France.
4. Department of Plant and Soil Science, University of Aberdeen, Aberdeen AB24 3FX, UK.
5. Alterra Green World Research, Wageningen, 6700 AC, Netherlands.