Description:
<jats:title>SUMMARY</jats:title><jats:p>The partial contribution by fixed nitrogen in N<jats:sub>2</jats:sub>‐fixing plants can be estimated if the <jats:sup>15</jats:sup>N natural abundance of non‐N<jats:sub>2</jats:sub>‐fixing reference species, which derive their N from the soil, deviates from that of atmospheric N<jats:sub>2</jats:sub> (the <jats:sup>15</jats:sup>N natural abundance method). Data from Tanzanian miombo woodland showed a significant difference (= 1.0‐2.5 δ<jats:sup>15</jats:sup>N%<jats:sub>0</jats:sub>) in <jats:sup>15</jats:sup>N 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 <jats:sup>15</jats:sup>N abundance as a marker of the ectomycorrhizal habit. It is suggested that the higher <jats:sup>15</jats:sup>N abundance of ectomycorrhizal tree species could result from differences in discrimination during uptake or from greater utilization of organic N.</jats:p>