This study investigates the relationship between soil fungal and bacterial communities and forest carbon storage across 238 forest inventory plots in 15 European countries. The researchers found that the composition and diversity of fungal species, particularly symbiotic endophytic and ectomycorrhizal fungi, are strongly linked to forest biotic conditions and tree growth rates and biomass carbon stocks. These fungal communities, especially those associated with trees, are significant predictors of forest carbon storage. The study also highlights the importance of fungal endophyte richness in promoting tree growth, which is a surprising finding given the focus on mycorrhizal fungi in forest ecosystems. Additionally, bacterial communities were more closely correlated with carbon stocks in the organic horizon, while fungal communities were more tightly linked to mineral horizon carbon stocks. The findings suggest that fungal composition is a strong bioindicator of overall forest carbon storage across Europe.This study investigates the relationship between soil fungal and bacterial communities and forest carbon storage across 238 forest inventory plots in 15 European countries. The researchers found that the composition and diversity of fungal species, particularly symbiotic endophytic and ectomycorrhizal fungi, are strongly linked to forest biotic conditions and tree growth rates and biomass carbon stocks. These fungal communities, especially those associated with trees, are significant predictors of forest carbon storage. The study also highlights the importance of fungal endophyte richness in promoting tree growth, which is a surprising finding given the focus on mycorrhizal fungi in forest ecosystems. Additionally, bacterial communities were more closely correlated with carbon stocks in the organic horizon, while fungal communities were more tightly linked to mineral horizon carbon stocks. The findings suggest that fungal composition is a strong bioindicator of overall forest carbon storage across Europe.