1. Centre for Plant Root Symbioses, Soil and Land Systems, School of Earth and Environmental Sciences, Waite Campus, The University of Adelaide, PMB1, Glen Osmond, South Australia, Australia 5064 (S.E.S., F.A.S.); and Plant Research Department, Risø National Laboratory, P.O. Box 49, DK-4000 Roskilde, Denmark (I.J.)