• 15 N natural abundance was examined to determine the fractionation associated with ammonium assimilation into fungi. The importance of fungi to the movement of nitrogen (N) within soils and into plants highlights the need to consider such fractionations in the interpretation of ecosystem 15 N distributions. • To separate the influence of physiological fractionations from N source preferences on fungal δ15 N, both ectomycorrhizal (EM) and saprotrophic (SAP) fungi were grown in liquid culture containing (NH4 )2 HPO4 of known δ15 N and harvested after 35 d. Time series experiments were employed for two taxa to determine how growth stage affects fractionation. • Mycelium δ15 N was positively associated with several measures of growth and no distinction was observed between EM and SAP fungi. Kinetic isotope exchange models explain the decrease in discrimination against 15 N at larger biomasses in terms of the amount of N utilized. The models indicate a fractionation of approx. -20‰, and suggest that assimilated N is excreted back into the medium during growth. • The observed fractionations at high and low N concentrations provide insight into the mechanisms that could drive fungal, plant and soil δ15 N. We suggest that detailed information on fungal life histories, territorial behaviours, and sink-source relationships in the fungus-plant-soil continuum could aid accurate ecological interpretations of δ15 N.
Keywords: ammonium assimilation; ectomycorrhizal; nitrogen; nitrogen recycling; saprotrophic; δ15N.