Climate Records, Isotopes, and C:N Stoichiometry Reveal Carbon and Nitrogen Flux Dynamics Differ Between Functional Groups of Ectomycorrhizal Fungi

A major functional division in ectomycorrhizal fungi is between taxa with hydrophobic ectomycorrhizae (strong proteolytic capabilities, deep nitrogen (N) acquisition, and extensive hyphal development) versus hydrophilic ectomycorrhizae (limited hyphal development, shallow N acquisition, and weak proteolytic capabilities). These two groups may differ in dynamic responses to recent plant carbon (C) fluxes and in N dynamics. Here, we measured sporocarp δ13C, δ15N, and C: N stoichiometry of ectomycorrhizal fungi from Betula-dominated stands in northern Finland from 1971 to 1983. We compared weather conditions for the 6 weeks prior to collection, hydrophobicity, C/N, and site-specific factors against the above parameters to test for (a) temporal dynamics of carbon fluxes from host plants to ectomycorrhizal fungi, (b) N dynamics of ectomycorrhizal fungi. Taxa primarily responded in δ13C and C/N to weather variability for the prior 3 weeks, indicating climatic conditions and plant C supply rapidly modulated fungal growth. However, hydrophilic taxa varied less overall in C/N than hydrophobic taxa and were less responsive in C/N to weather variability. Compared to hydrophilic taxa, species with hydrophobic ectomycorrhizae were low in δ13C and high in δ15N, with δ15N and C/N negatively correlated, particularly at low δ13C. This presumably reflected the partitioning of 13C-enriched and 15N-depleted structural carbohydrates and chitin into the extensive belowground mycelia, limited influx of soil-derived N during sporocarp formation (semi-closed system), and the subsequent formation of 13C-depleted and 15N-enriched sporocarps. Functional differences in C and N dynamics among ectomycorrhizal taxa were deduced from weather records and sporocarp δ13C, δ15N, and C/N measurements.