An experiment was performed to find out whether ectomycorrhizal (ECM) fungi alter the nitrogen (N) isotope
composition, δ15N, of N during the transport of N from the soil through the fungus into the plant. Non-
mycorrhizal seedlings of Pinus sylvestris were compared with seedlings inoculated with either of three ECM fungi,
Paxillus involutus, Suillus bovinus and S. variegatus. Plants were raised in sand in pots supplied with a nutrient
solution with N given as either NH4+ or NO3−.
Fractionation against 15N was observed with both N sources; it
decreased with increasing plant N uptake, and was larger when NH4+ was the source. At high ratios of
Nuptake/Nsupplied there was no (NO3−),
or little (NH4+), fractionation. There seemed to be no difference in
fractionation between ECM and non-mycorrhizal plants, but fungal rhizomorphs were sometimes enriched in
15N (up to 5‰ at most) relative to plant material; they were also enriched relative to the N source. However, this
enrichment of the fungal material was calculated to cause only a marginal decrease (−0.1‰ in P. involutus) in
δ15N of the N passing from the substrate through the fungus to the host, which is explained by the small size of the
fungal N pool relative to the total N of the plant, i.e. the high efficiency of transfer. We conclude that the relatively
high 15N abundance observed in ECM fungal species should be a function of fungal physiology in the ECM
symbiosis, rather than a reflection of the isotopic signature of the N source(s) used. This experiment also shows
that the δ15N of plant N is a good approximation of δ15N of the available N source(s), provided that N is limiting
growth.