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

An experiment was performed to find out whether ectomycorrhizal (ECM) fungi alter the nitrogen (N) isotope composition, delta(15)N, of N during the tr ansport of N from the soil through the fungus into the plant. Non-mycorrhiz al seedlings of Pinus sylvestris were compared with seedlings inoculated wi th either of three ECM fungi, Paxillus involutus, Suillus bovinus and S. va riegatus. Plants were raised in sand in pots supplied with a nutrient solut ion with N given as either NH4+ or NO3-. Fractionation against N-15 was obs erved 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 n o difference in fractionation between ECM and non-mycorrhizal plants, but f ungal rhizomorphs were sometimes enriched in N-15 (up to 5%, at most) relat ive to plant material; they were also enriched relative to the N source. Ho wever, this enrichment of the fungal material was calculated to cause only a marginal decrease (-0.1 parts per thousand in P. involutus) in delta(15)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 r elatively high N-15 abundance observed in ECM fungal species should be a fu nction of fungal physiology in the ECM symbiosis, rather than a reflection of the isotopic signature of the N source(s) used. This experiment also sho ws that the delta(15)N of plant N is a good approximation of delta(15)N Of the available N source(s), provided that N is limiting growth.