Nitrogen isotope fractionation during nitrogen uptake by ectomycorrhizal and non‐mycorrhizal <i>Pinus sylvestris</i>

<jats:p>An experiment was performed to find out whether ectomycorrhizal (ECM) fungi alter the nitrogen (N) isotope composition, δ<jats:sup>15</jats:sup>N, of N during the transport of N from the soil through the fungus into the plant. Non‐ mycorrhizal seedlings of <jats:italic>Pinus sylvestris</jats:italic> were compared with seedlings inoculated with either of three ECM fungi, <jats:italic>Paxillus involutus</jats:italic>, <jats:italic>Suillus bovinus</jats:italic> and <jats:italic>S. variegatus</jats:italic>. Plants were raised in sand in pots supplied with a nutrient solution with N given as either NH<jats:sub>4</jats:sub><jats:sup>+</jats:sup> or NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>. Fractionation against <jats:sup>15</jats:sup>N was observed with both N sources; it decreased with increasing plant N uptake, and was larger when NH<jats:sub>4</jats:sub><jats:sup>+</jats:sup> was the source. At high ratios of N<jats:sub>uptake</jats:sub>/N<jats:sub>supplied</jats:sub> there was no (NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>), or little (NH<jats:sub>4</jats:sub><jats:sup>+</jats:sup>), fractionation. There seemed to be no difference in fractionation between ECM and non‐mycorrhizal plants, but fungal rhizomorphs were sometimes enriched in <jats:sup>15</jats:sup>N (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 <jats:italic>P. involutus</jats:italic>) in δ<jats:sup>15</jats:sup>N 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 <jats:sup>15</jats:sup>N 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 δ<jats:sup>15</jats:sup>N of plant N is a good approximation of δ<jats:sup>15</jats:sup>N of the available N source(s), provided that N is limiting growth.</jats:p>