Forest soils constitute a major carbon pool in the context of the global carbon cycle. Although ectomycorrhizal fungi (EMF) are responsible for a large part of the input of carbon into this pool, information on the transformation of EMF biomass into soil organic matter (SOM) is scarce. In order to quantitatively analyze the contribution of EMF biomass to SOM formation, we incubated a typical forest arenosol with 13C-labeled fungal biomass from the ectomycorrhizal basidiomycete Laccaria bicolor in a closed bioreactor system for 231 d, enabling the establishment of a full mass balance for the turnover. rDNA fragments specific for the genus Laccaria disappeared after 28 d, indicating death and decay of the organism. The amount of 13C PLFA as a proxy for living biomass decreased to roughly 10% of the initial value in the course of the experiment, again suggesting efficient disintegration of the fungal organism. After 231 d, however, 63.5% of the initially added 13C had been converted to SOM and 52% belonged to the non-living fraction. The analysis of 13CO2 formation (36.5% of the initially added label) suggested two-pool exponential kinetics with k1 = 0.0615 d−1 for the easily degradable and k2 = 0.0005 d−1 for the stable fraction (pool sizes 74% and 26% of the initially added label, respectively). When compared to the degradation of bacterial biomass in agricultural soils, the pool of readily degradable fungal biomass was considerably smaller and the half-lives of both pools (readily degradable and persistent C) were markedly higher for fungal biomass. In summary, our data allow the conclusion that EMF biomass is a significant source of SOM in forest ecosystems. •A full mass balance for EMF-derived 13C in a forest soil was set up.•After 231 d, 64% of the added C remained in soil and 36% were mineralized.•The degradation kinetics differed from studies with bacterial derived biomass.•EMF necromass is a potential significant contributor to SOM formation.