• 15N;
  • ectomycorrhiza;
  • nitrogen fixation


The partial contribution by fixed nitrogen in N2-fixing plants can be estimated if the 15N natural abundance of non-N2-fixing reference species, which derive their N from the soil, deviates from that of atmospheric N2 (the 15N natural abundance method). Data from Tanzanian miombo woodland showed a significant difference (= 1.0-2.5 δ15N%0) in 15N abundance between ectomycorrhizal and vesicular-arbuscular (VA) mycorrhizal reference species. This finding casts doubt upon arbitrary selection of reference species, and also raises the possibility of using 15N abundance as a marker of the ectomycorrhizal habit. It is suggested that the higher 15N abundance of ectomycorrhizal tree species could result from differences in discrimination during uptake or from greater utilization of organic N.