1. Institute of Environmental Geochemistry, University of Heidelberg, INF 236, 69120 Heidelberg, Germany, Department of Ecology and Environmental Sciences, Umeå University, 907 42 Umeå, Sweden, Department of Soil Science and Agricultural Chemistry, Faculty of Biology, University of Santiago, E-15782 Santiago de Compostela, Spain, and St. Croix Watershed Research Station, Science Museum of Minnesota, St. Croix, Minnesota 55047