Probing ectomycorrhizal function belowground: Nitrogen isotopes in ectomycorrhizal fruitbodies correspond to belowground exploration types

Background/Question/Methods               Nitrogen isotope measurements on ectomycorrhizal fungi hold considerable promise for insights into plant nitrogen sources and patterns of nitrogen and carbon cycling in different fungal taxa.   Although nitrogen isotope values (δ¹⁵N) are generally higher in ectomycorrhizal fungi than in their plant hosts, the wide variability in δ¹⁵N among fruitbodies (mushrooms) 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 (long-distance organs for fungal transport) in ectomycorrhizal species from surveys at four sites in temperate and boreal coniferous forests that varied in nitrogen availability (low nitrogen availability: Åheden, northern Sweden and Deer Park, Pacific Northwest, USA;  high nitrogen availability: Stadsskogen, central Sweden and Hoh, Pacific Northwest, USA).   Fungi were classified into 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) and exploration types of low biomass [medium-distance smooth (e.g., Amanita), short-distance (e.g., Inocybe), and contact (e.g., Hygrophorus)].       Results/Conclusions         Fungi with exploration types of high biomass were 4-7‰ more enriched in ¹⁵N than fungi with exploration types of low biomass.  High-biomass exploration types were more prevalent at sites of low 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.   Analytical equations of nitrogen isotopes indicate that fungal nitrogen should be treated as two separate pools, a mobile pool and an immobile pool, with fruitbody nitrogen derived from a mobile pool of stored protein and the immobile pool consisting of cross-linked proteins, carbohydrates, and amino sugars of fungal cell walls.   Within exploration types, fruitbodies from fungi colonizing lower soil horizons were enriched in ¹⁵N relative to those colonizing upper soil horizons.  The concept of exploration types appears  a useful classification tool for assessing the functioning of different ectomycorrhizal taxa belowground when combined with the quantitative information provided by nitrogen isotopes.