Isotopic analysis of protein and structural material of sporocarps reveals fungal C and N sources

Background/Question/Methods

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.

Results/Conclusions

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.