1. Aarhus Univ., National Environmental Research Institute; Dep. of Freshwater Ecology; Vejlsoevej 25 DK8600 Silkeborg Denmark
2. Aarhus Univ.; Faculty of Agricultural Sciences, Dep. of Agroecology and Environment; Blichers Allé 20, P.O. Box 50 DK-8830 Tjele Denmark
3. MTT Agrifood Research Finland; Plant Production Research, Soil and Plant Nutrition; E-talo FI-31600 Jokioinen Finland
4. Dep. of Basic Sciences and Environment, Faculty of Life Sciences; Univ. of Copenhagen; Thorvaldsensvej 40 DK-1871 Frederiksberg C Denmark