1. Institute of Plant Nutrition and Soil Science & Project Centrum Ecosystem Research, University Kiel, Olshausenstraße 40–60, W‐2300 Kiel 1, Germany
2. Fachhochschule Fresenius, Department of Trace Analysis, Dambachtal 20, W‐6200 Wiesbaden, Germany
3. Institute of Biophysics and Physical Biochemistry, University Regensburg, Postfach, W‐8400 Regensburg, Germany