Nitrogen isotopes in ectomycorrhizal sporocarps correspond to belowground exploration types

Abstract

Nitrogen isotope values (δ15N) are higher in ectomycorrhizal fungi than in their plant hosts but the wide variability in δ15N among sporocarps of different fungal taxa is unexplained. We propose that fungal δ15N reflects sequestration of fungal nitrogen to build fungal biomass, and should accordingly reflect fungal exploration strategies and hyphal properties. To test this, we compared δ15N to exploration types, hyphal hydrophobicity, and the presence of rhizomorphs in ectomycorrhizal species from surveys at four sites in temperate and boreal coniferous forests. Fungi with exploration types of high biomass, such as long-distance (e.g., Suillus), medium-distance mat (e.g., Hydnellum), and medium-distance fringe (e.g., Cortinarius) were 4–7‰ more enriched in 15N than fungi with exploration types of low biomass [medium-distance smooth (e.g., Amanita), short-distance (e.g., Inocybe), and contact (e.g., Hygrophorus)]. High biomass types comprised 79% (Aheden, northern Sweden), 65% (Deer Park, Pacific Northwest, USA), 45% (Stadsskogen, central Sweden), and 39% (Hoh, Pacific Northwest, USA) of ectomycorrhizal species, with these types more prevalent at sites of lower nitrogen availability. Species with hydrophobic hyphae or with rhizomorphs were 3–4‰ more enriched in 15N than taxa with hydrophilic hyphae or without rhizomorphs. The consistency of these patterns suggest that δ15N measurements could provide insights into belowground functioning of poorly known taxa of ectomycorrhizal fungi and into relative fungal biomass across ectomycorrhizal communities.