Rapid incorporation of carbon from ectomycorrhizal mycelial necromass into soil fungal communities

Ectomycorrhizal mycelial necromass is an important source of carbon for free-living microorganisms in forest soils, yet we know little either of its fate when it enters soil or of the identity of microbes that are able to utilise mycelium as their energy source. Here we used ¹³C-labelled mycelium of the ectomycorrhizal fungus Pisolithus microcarpus in laboratory incubations in combination with DNA-stable isotope probing (SIP) to determine the identity of functionally active soil fungi that can utilise dead mycelium. We also used solid-state nuclear magnetic resonance (NMR) spectroscopy to detect parallel changes in the abundance of key biochemical constituents of soil. A decrease in bulk soil ¹³C concentration together with rapid loss of glycogen and chitin-glucan during the 4 week incubations suggested that dead mycelium was rapidly turned over. Further, ¹³C was incorporated into fungal DNA within 7 days of addition to soil. DNA-SIP also revealed a dynamic community of functionally active soil fungi. By applying DNA-SIP and NMR in parallel, our data show that carbon from decaying ectomycorrhizal mycelium is rapidly transformed and incorporated into free-living soil fungi. This finding emphasises that dead extra-matrical mycelium is an important source of labile carbon for soil microorganisms.