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Fungi are one of the key components in the terrestrial carbon cycle, performing a significant part of the decomposition process. Without them, the degradation of organic matter would be much slower and more limited. While fungi consume large amounts of C in forested ecosystems, quantifying how much this contributes to the overall decomposition-derived CO2 is difficult. Here, we show that radiocarbon can be a useful tool both for understanding more about the activity of these fungal species and how they contribute to the transit time of C in forested ecosystems. Since the cessation of atmospheric nuclear testing, the declining level of atmospheric 14C has made it possible to date when carbon was fixed from the atmosphere with even annual precision (Hua et al., 2022). Atmospheric 14C is naturally incorporated into recent biological materials through photosynthesis, naturally labelling them and indicating how long-ago C in different tree organs and tissues was fixed. Radiocarbon measurements have shown that some mycorrhizal fungal species use carbon that is quite recently fixed, while saprophytic fungi that mainly consume dead organic matter incorporate C fixed years to decades previously in their tissues (Hobbie et al., 2002).For our studies, we selected saprobiotic, wood-decomposing fungi from forests in Thuringia, Germany. Our results using 14C analysis by accelerator mass spectrometry show that even when growing on living trees, these fungi use carbon fixed up to ~30 years ago to produce their fruiting bodies. Fungi sampled on dead trees can use carbon fixed on average 40 years ago to create their fruiting bodies. For four selected trees, we compared the age of the trees estimated from tree rings with the mean radiocarbon age of xylophagous fungi. It was determined that the 14C age of the fungi was closely aligned with the mean age of the tree, thereby indicating a widespread infection within the tree prior to the formation of a fruiting body. These results provide information on the age and the physical location of C substrates used by wood-decomposing fungi and provide a tracer to indicate the importance of decades-old wood decomposition to the overall transit time of C in forested ecosystems.References Hobbie, E. A., Weber, N. S., Trappe, J. M., & Van Klinken, G. J. (2002). Using radiocarbon to determine the mycorrhizal status of fungi. New Phytologist, 156(1)Hua, Q., Turnbull, J. C., Santos, G. M., Rakowski, A. Z., Ancapichún, S., De Pol-Holz, R., Hammer, S., Lehman, S. J., Levin, I., Miller, J. B., Palmer, J. G., & Turney, C. S. M. (2022). Atmospheric radiocarbon for the period 1950–2019. Radiocarbon