1. Dep. of Plant Ecology; Justus-Liebig Univ.; Heinrich-Buff-Ring 26-32 D-35392 Giessen Germany
2. Institute of Soil Biogeochemistry, Martin-Luther University; Halle-Wittenberg, Von-Seckendorff-Platz 3 06120 Halle (Saale) Germany
3. School of Biology and Environmental Sciences, Univ. College Dublin; Dublin Ireland