Fungi perform keystone roles in terrestrial nutrient cycles through uptake of mineral nutrients by mycorrhizae and return of elements by saprotrophic decomposition of detritus. Linking these ecological roles to specific microbial organisms is necessary to refine understanding of biogeochemical cycling and fungal ecophysiology. We analyzed published and novel fungal nitrogen (δ¹⁵N) and carbon (δ¹³C) stable isotope values to test if global patterns follow theoretical predictions and could be used to reliably identify ecological roles of macrofungi. Correlation analyses with climate and latitude indicated that systematic variation among both isotopes required site normalization prior to conducting discriminant multivariate analyses.
Results/Conclusions

A discriminant multivariate analysis verified collector based categorization of ectomycorrhizal (ECM) and saprotrophic (SAP) fungi in 92.5% of species and predicted probabilistic classifications for fungi of unknown trophic status despite significant background isotope variation. Fungal δ¹³C was best correlated with latitude (ECM r²=0.54, SAP r²=0.70) and mean annual temperature (ECM r²=0.26, SAP r²=0.66) whereas fungal δ¹⁵N was correlated with mean annual temperature in ECM fungi only (r²=0.28). The difference between ECM and SAP (Δ_ECM-SAP) δ¹⁵N and δ¹³C values were of comparable magnitudes (6‰ and -2.2‰) across sites despite broad ranges in climate and soil type. The consistent isotopic divide among trophic groups suggests that fungal physiology appears to exert greater influence on sporocarp δ¹⁵N than source-N and that C-source (photosynthate in ECM and detritus in SAP) isotope effects are similar across the globe.