Saprotrophic-mycorrhizal divide in stable isotope composition throughout the whole fungus: from mycelium to hymenophore.

Mycorrhizal and saprotrophic macromycetes contribute strongly to the carbon and nitrogen cycles of forest ecosystems, often studied by tracing stable isotope composition of carbon and nitrogen. The phenomenon of the saprotrophic-mycorrhizal divide highlights the difference in the stable isotope composition of fruiting bodies of mycorrhizal and saprotrophic fungi. Much less is known about the isotopic composition of the mycelium, which plays an important role in the formation of the soil organic matter and fuels the fungal trophic channel in soil food webs. In this study, we assessed whether the saprotrophic-mycorrhizal divide in the natural δ¹³С and δ¹⁵N values can be traced throughout entire fungal organisms. This hypothesis was tested using 16 species of ectomycorrhizal and six species of saprotrophic basidiomycetous fungi. We showed that not only fruiting bodies, but also the mycelium of ectomycorrhizal and saprotrophic fungi differs in the δ¹³C and δ¹⁵N values. In both ectomycorrhizal and saprotrophic fungi, the δ¹³C and δ¹⁵N values increased from mycelium to hymenophores and correlated positively with the total N content in the corresponding tissues. The differences between ectomycorrhizal and saprotrophic mycelium can be used to reconstruct the fungal-driven belowground carbon and nitrogen allocation, and the contribution of saprotrophic and mycorrhizal fungi to soil food webs.