95th ESA Annual Meeting (August 1 -- 6, 2010)

COS 71-2 - Variation in soil nitrogen availability and substrate use affect the 15N signatures of trees, saprotrophic, and ectomycorrhizal fungi

Wednesday, August 4, 2010: 1:50 PM
411, David L Lawrence Convention Center
Lucas E. Nave1, Knute Nadelhoffer2, James Le Moine3 and Marilynn Smith1, (1)University of Michigan Biological Station, Pellston, MI, (2)Director, UM Biological Station (Pellston, MI), University of Michigan, Ann Arbor, MI, (3)University of Michigan

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.