1. Centre for Ecology and Hydrology, Bush Estate, Penicuik, Midlothian, EH26 0QB U.K., School of Chemistry, University of Edinburgh, King's Buildings, West Mains Road, Edinburgh EH9 3JJ, U.K., and Institute for Soil Ecology, GSF National Research Center for Environment and Health, Neuherberg, D-85764 Oberschleissheim, Germany