1. a Institute of Plant Nutrition and Soil Science , University of Kiel , Olshausenstrasse 40–60, Kiel , D‐24118 , Germany
2. c Nedre Toppenhaug 100, Baerums Verk , N‐1353 , Norway
3. b Ecology Centre Kiel , University of Kiel , D‐24110, Schauenburger Strasse 112, Kiel , Germany