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

To quantify and characterize N and C isotopic fractionation effects due to fungal transformation of organic substrates in forest ecosystems, we perfor med a field study in California and a meta-analysis of three additional stu dies conducted by others across the Northern Hemisphere. Basidiomycete fung al 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 distinct ion between ectomycorrhizal and saprotrophic basidiomycetes was strongly su pported, particularly when dual isotope analyses were performed. This diffe rentiation, which we call the "EM-SAP Divide" holds for studies within a re stricted ecosystem, but becomes less distinct over larger geographical regi ons, removing the rationale for using direct isotopic values from single sp ecimens as diagnostic of ecophysiological role. For C, the EM-SAP Divide se ems to reflect substrate effects, potentially due to differential access to recently synthesized versus recycled organic compounds, rather than distin ct physiological pathways. Once substrate and ecophysiological role effects are removed, our meta-analysis suggests the existence of more than one mec hanism causing C fractionations in fungi which is found equally in ectomyco rrhizal and saprotrophic fungi. Similarly, a multimodal distribution of del ta N-15 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 b ased on natural isotopic distributions of C and N. We call into question th e current practice of using direct isotopic measurements to make statements about trophic relationships of fungi in the absence of other supporting ev idence.