N natural abundance as a possible marker of the ectomycorrhizal habit of trees in mixed African woodlands.

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