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Controls of Isotopic Patterns in Saprotrophic and Ectomycorrhizal Fungi
Hobbie, E. A., F. S. Sanchez, AND P. T. RYGIEWICZ. Controls of Isotopic Patterns in Saprotrophic and Ectomycorrhizal Fungi. SOIL BIOLOGY AND BIOCHEMISTRY. Elsevier Science Ltd, New York, NY, 48:60-68, (2012).
The research reported in this manuscript was conducted by a National Academy of Sciences, NRC Post-Doctoral Fellow, and a fellow of the Scientist Committee of NATO, both of whom were located at WED. The research was also funded by the National Science Foundation, Environmental Biology. The research contributed to the WED project Extrapolating Anthropogenic Stress Effects: Individuals to Forests, Ecosystems, and Regions (INFER). One overarching objective of INFER was to develop the capabilities to scale ecosystem processes across gradients of space, time and biological complexity to inform predictions of consequences of anthropogenic stresses. The overall objective of this collaborative research was to use stable isotope signatures of nitrogen and carbon to provide a mechanistic understanding of the taxanomic variation in how a dominant (in terms of biomass) functional group of soil microbiota (higher fungi) assimilate and partition nitrogen and carbon across a large geographic range within the PNW. The geographic range captured a substantial portion of differences in climate, water dynamics and soils found in Oregon. Isotope signatures of various parts of fruit bodies representative of two major fungal life strategies, i.e., ectomyorrhizal (symbionts with tree roots) and saprotophic (decomposer), suggest that the caps rather than the stipes (stems) are more sensitive indicators of acquisition and use of organic versus inorganic nitrogen sources within forested ecosystems. Conventional understanding has been that trees acquire the vast majority of their nitrogen after decomposition proceeds to production of inorganic forms of nitrogen. These results on fungi suggest that multiple pathways, involving multiple forms of nitrogen, are operating simultaneosuly in forest soils. Understanding the taxanomic variation in the relative contribution of various forms of nitrogen to the nutrient cycle has implications for understanding the potential consequences of anthropogenic stresses. This especially would be the case should stresses alter the community composition of ecosystems, or affect the stoichiometric balance between the carbon and nitrogen cycles, as has been shown to occur under increased atmospheric concentrations of CO2 and possible associated altered future climate scenarios.
Isotopes of nitrogen (δ15N) and carbon (δ13C) in ectomycorrhizal and saprotrophic fungi contain important information about ecological functioning, but the complexity of physiological and ecosystem processes contributing to fungal carbon and nitrogen dynamics has limited our ability to explain differences across taxa. Here, we measured δ15N and δ13C in needles, litter, soil, wood, fungal caps, and fungal stipes at numerous forested sites in Oregon, USA to determine how functional attributes and biochemical processes may influence isotopic values. Ectomycorrhizal fungi were classified by hydrophobicity of ectomycorrhizae and by patterns of hyphal exploration; saprotrophic fungi were classified into wood decay and litter decay fungi. For δ15N, caps of hydrophobic taxa averaged 8.6‰, hydrophilic taxa 3.2‰, and saprotrophic taxa 0.5‰, whereas needles averaged 3‰ and soil at 5–12 cm averaged 2‰. Caps were higher in δ15N, δ13C, %N, and %C than stipes by 1.7‰, 0.6‰, 1.75%, and 2.61%, respectively, presumably because of greater protein content in caps than stipes. Isotopic enrichment of caps relative to stipes was greater in hydrophobic taxa (3.1‰ for 15N and 0.8‰ for 13C) than in hydrophilic taxa (1.1‰ for 15N and 0.5‰ for 13C). In multiple regressions, 45% of variance in δ15Ncap–stipe and 30% of variance in δ13Ccap–stipe was accounted for by various elemental, isotopic, and categorical variables. We estimated that fungal protein was enriched in 15N relative to fungal chitin by 15‰ in hydrophobic taxa and by 7‰ in hydrophilic taxa. Fungal protein was enriched in 13C by 4.2 ± 0.5‰ relative to carbohydrates. Isotopic signatures of sources and isotopic fractionation during metabolic processing influence both isotopic patterns of sporocarps and the isotopic partitioning between caps and stipes; functional groups differed in processing of both nitrogen isotopes and carbon isotopes.