Mean δ15N of whole-plants of lettuce and barley varied by [les ]3‰ when given a chemically and isotopically uniform N source. This variation was related to the presence, absence and species of arbuscular mycorrhizal (AM) fungi and to external N concentration. A highly AM-susceptible plant (lettuce) responded to treatments differently than a less susceptible one (barley). The largest change in whole plant δ15N was related to the experimental combination most likely to be found in field conditions: species of fungus interacting with varying external concentrations of N.

The mechanisms underlying observed variations of plant δ15N are not understood, nor can they be approached directly using δ15N. However, descriptive data, such as presented here, are important to the development of a mechanistic model, because they suggest relationships for future research, using inter alia15N-enriched tracers. They also confirm that plant N sources, cannot be identified using plant δ15N, even when the type of mycorrhizal association (endo vs. ecto) is known.