| In this thesis, I characterize the effect of various basidiomycete fungi on the natural distribution of stable carbon (C) and nitrogen (N) isotopes. Fungi are ubiquitous microbes responsible for most decomposition, as well as many other functions taking place in terrestrial ecosystems, so that most ecosystem processes can be conceived as having one or more stages mediated by fungal interfaces.; Using field and laboratory descriptions, I define boundary values for fungal fractionation effects of C and N stable isotopes in conifer-related ecosystems, and partition fractionation effects due to substrate preferences versus intrinsic, physiological-based fractionations. Preferences for a particular C-source are able to account for a large extent of the variability observed in δ13C-values of ectomycorrhizal (EM) versus saprotrophic (SAP) fungi in the field, though an additional signal remained in the isotopic distributions even after substrate preferences were taken into account. Laboratory studies delineate two major physiological C pathways underlying 13 C-enrichments in all fungi studied relative to their respective substrates. The two metabolic processes exhibit contrasting fractionation patterns during C uptake. Further, expression of these two C fractionating mechanisms is sensitive to the growth stage of the fungus, the concentration of growth factors in the medium, and the distribution of isotopic species within the C-source. While it is unclear what the impact of substrate preferences are on the fungal δ 15N-values in the field due to a difficulty in knowing the substrate, laboratory studies show that significant fractionations occur during the uptake of ammonium into fungal cells. The degree of 15N-depletion in fungal biomass is associated with the different growth stages of the fungus and dependent on the net transformation of N into the biomass. I indicate that observed 15N-enrichments in fungal sporocarps relative to bulk plants and soils likely is the result of an additional fractionation of stable N isotopes during sporocarp formation.; I extend my findings to the interpretation of C and N isotope measurements of various ecosystem components in terrestrial ecosystems. A framework to address when C and N isotopic fractionations may be relevant in such systems is developed. (Abstract shortened by UMI.)... |
