Elucidating the nutritional status of tropical fungi using dual isotopes: Implications for ecosystem nutrient dynamics

Isotopic signatures (δ¹³C, δ¹⁵N) obtained from fungal fruiting bodies have been previously shown to segregate ectomycorrhizal (ECM) from saprotrophic (SAP) macrofungi in temperate coniferous and broadleaf forests.  Here we present a discriminant meta-analysis of 674 observations to test for global synonymy in ECM and SAP isotope patterns.  This “trained” model was then used to discriminate the trophic status of 49 fungi collected from a unique Guyanese tropical rainforest featuring a diverse assemblage of ECM fungi associated with Dicymbe corymbosa (Caesalpiniaceae).  The discriminant analysis accurately separated field-verified ECM from SAP species, assigned trophic classifications for unknown fungi, and provided a metric of observer-based classification error.  The tropical ECM species’ mean isotope values were in agreement with temperate ecosystems, with ECM fungi consistently more enriched in ¹⁵N and more depleted in ¹³C than SAP fungi.  The ¹⁵N pattern of ECM enrichment and corresponding host plant depletion are best explained by isotopic fractionation during the formation of N transfer compounds by ECM fungi.  This physiological mechanism, if operational in the Dicymbe-ectotrophic system, implies that ECM fungi are important for tree nitrogen (N) nutrition even under the potentially phosphorus (P) limiting conditions found in the oligotrophic soils of the Guiana Shield.  This hypothesis suggests that mycorrhizal nutrient acquisition does not necessarily follow a strict economic model where only the most plant limiting nutrient is supplied by ECM.  Instead, both N and P are likely delivered simultaneously by ECM fungi in response to plant limitation of either mineral nutrient.