While fungal composition has been linked to soil carbon at global scales, these patterns are often difficult to disentangle from broad climatic gradients and species range limits. To address this limitation, we tested which aspects of ectomycorrhizal community structure explain soil carbon variation within a single temperate woodland landscape, where climate and species pools are relatively constant. Across 189 plots, we sequenced >3,500 live ectomycorrhizal root tips, linking root-level morphological traits to measured soil carbon stocks. Community composition and foraging traits—rather than diversity or abundance—showed the strongest and most consistent associations with carbon after accounting for soil chemistry, topography, and vegetation. Several ectomycorrhizal taxa also emerged as indicators of carbon-rich soils, offering finer resolution than conventional forest classifications. Our findings highlight the functional importance of ectomycorrhizal communities for soil carbon accumulation and suggest that integrating fungal community data could improve carbon monitoring and management at landscape scales.