Studies of the stable isotope composition (13C and 15N) of fungal sporocarps have produced various explanations for variation in isotopic signatures of these conspicuous participants in forest C and N cycling. Mechanisms responsible for the 15N enrichment of ectomycorrhizal (ECM) sporocarps are particularly relevant from a biogeochemical perspective, and probably include variation in substrate use, mycelial development and morphology, and N transfer to phytobiont hosts. We measured soil N availability using ion exchange resin incubations, fungal sporocarp 15N and 13C abundances, and percent N and 15N contents of tree foliage in an oligotrophic temperate forest to test whether 15N signatures and trophic strategies of fungal sporocarps (saprotrophic and ECM) vary with soil N availability. We also investigated soil N availability as a driver of variation in foliar 15N signatures and ECM C and N transfers as quantified according to a published mass-balance model.
Our data on saprotrophic fungi showed evidence for two divergent trophic groups with unique responses to increasing soil N availability. The first group, typified by Armillaria mellea, used isotopically depleted N to decompose relatively 13C-enriched organic C fractions. Fungal biomass from this group showed increasing 15N enrichment along a gradient of increasing soil N availability. The second group was exemplified by Leotia lubrica, which used relatively 13C-depleted, recent-C inputs to forest floors to access 15N-enriched, humic N fractions. Fungal sporocarps from this group became progressively 15N-depleted as soil N availability increased. ECM sporocarps were consistently more 15N-enriched than these co-occurring saprotrophs but nonetheless showed the same relationships between 15N enrichment and soil N availabilty. ECM sporocarps from the genera Amanita, Laccaria, Lactarius, and Russula all showed greater 15N enrichment with increasing soil N availability, while Boletus spp. and Leccinum spp. showed progressive 15N-depletion along the same gradient. Foliage from two of the dominant tree species at the site, Acer rubrum and Quercus rubra, showed 15N enrichment with increasing soil N availability, and isotopically heavier foliage also had higher N content. N transfers from ECM hyphae to Q. rubra appeared to be relatively less important as an N source as soil N availability increased, despite an increasing proportion of hyphal N being transferred at higher levels of soil N availability. C allocation to ECM hyphae declined nonlinearly with increasing soil N availability. These relationships between soil N availability and ECM C and N transfers are deserving of further study by independent methods.