Ecophysiology of ¹³C and ¹⁵N isotopic fractionation in forest fungi and the roots of the saprotrophic-mycorrhizal divide

Abstract

To quantify and characterize N and C isotopic fractionation effects due to fungal transformation of organic substrates in forest ecosystems, we performed a field study in California and a meta-analysis of three additional studies conducted by others across the Northern Hemisphere. Basidiomycete fungal biomass was consistently enriched for the heavier isotope for C relative to substrate and either enriched or depleted for N relative to atmospheric N. Extent and pattern of fractionation was very variable, but the distinction between ectomycorrhizal and saprotrophic basidiomycetes was strongly supported, particularly when dual isotope analyses were performed. This differentiation, which we call the "EM-SAP Divide" holds for studies within a restricted ecosystem, but becomes less distinct over larger geographical regions, removing the rationale for using direct isotopic values from single specimens as diagnostic of ecophysiological role. For C, the EM-SAP Divide seems to reflect substrate effects, potentially due to differential access to recently synthesized versus recycled organic compounds, rather than distinct physiological pathways. Once substrate and ecophysiological role effects are removed, our meta-analysis suggests the existence of more than one mechanism causing C fractionations in fungi which is found equally in ectomycorrhizal and saprotrophic fungi. Similarly, a multimodal distribution of δ¹⁵N values suggests that physiological effects may play a much stronger influence on N natural isotopic distributions in fungi. Our meta-analysis provides a firm statistical base to evaluate fungal ecological statements based on natural isotopic distributions of C and N. We call into question the current practice of using direct isotopic measurements to make statements about trophic relationships of fungi in the absence of other supporting evidence.