Isotopic analysis of protein and structural material of sporocarps reveals fungal C and N sources
Functional attributes of fungi have been difficult to study under field conditions. Isotopic labels introduced in the Duke Free Air CO2 enrichment (FACE) experiment provided a unique opportunity to study the sources of C and N for ectomycorrhizal and saprotrophic fungi in a loblolly pine plantation without disturbance. Soils were labeled once with trace amounts of 15N in 2003 and CO2-treated plots were fumigated with 13C-depleted CO2 from 1997 to 2010. Sporocarps were collected from ambient and FACE treatment plots during and after CO2 enrichment. Stable isotope analysis of structural carbon and protein from sporocarps were used to determine whether fungi were receiving carbon from recent plant photosynthate, plant litter or old organic nitrogen and whether fungi were receiving nitrogen from surficial or deep soil pools.
We tracked 13C-depleted CO2 into saprotrophic and ectomycorrhizal fungi along with the gradual spread of 15N applied to litter horizons into deeper soil horizons. Bulk saprotrophic and ectomycorrhizal fungi had lower δ13C in elevated CO2 than in ambient treatments. Two years after CO2 treatments had ended, saprotrophic sporocarps still had low δ13C, whereas ectomycorrhizal sporocarps increased to pretreatment δ13C values. Low δ13C values also persisted 2 years post-CO2 treatment in protein of hydrophobic ectomycorrhizae, but not structural material or hydrophilic ectomycorrhizae. Taxa with hydrophobic ectomycorrhizae also had relatively high 15N enrichment in protein, and to a lesser extent bulk material, nine years after introducing the 15N label. The persistently lower δ13C and higher 15N enrichment in protein from taxa with hydrophobic ectomycorrhizae compared to taxa with hydrophilic ectomycorrhizae indicate that the former fungi mine old, deeper organic material for N and protein C and while using recent plant photosynthate for structural material. Hydrophilic fungi use recent plant photosynthate and C and N from surficial soil pools and saprotrophic fungi use recent plant litter and surficial soil C and N. Patterns in C and N acquisition indicated that hyphal exploration strategies and capabilities to access different soil resources were functionally linked.