Nitrogen isotope fractionation during nitrogen uptake by ectomycorrhizal and non-mycorrhizal Pinus sylvestris

Abstract

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 N_uptake/N_supplied 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.