1. School of Biological Sciences, Sutton Bonington Campus, University of Nottingham, Loughborough, Leicestershire LE12 5RD, U.K. National Radiological Protection Board, Chilton, Didcot, Oxfordshire OX11 ORQ, U.K. Laboratory of Soil Fertility and Soil Biology, Faculty of Applied Biological and Agricultural Sciences, K. U. Leuven, K. Mercierlaan, 92, B 3001 Heverlee, Belgium