Ectomycorrhizal community composition and extramatrical hyphal proportion predict soil carbon stocks at the landscape scale

Abstract 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 constraint, 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 identified fungi from over 3500 ectomycorrhizal roots, and characterised composition, diversity, abundance, and morphological traits (hyphae and rhizomorphs). We related these to soil carbon stocks using single‐predictor models (fungi alone) and a multiple‐predictor model that adjusted for soil chemistry, topography and woodland type. Community composition and extramatrical hyphal proportion—rather than diversity or abundance—showed the strongest and most consistent associations with carbon. In single‐predictor models, conifer‐associated ectomycorrhizal communities were correlated with larger carbon stocks, consistent with continental‐scale patterns. However, after accounting for soil chemistry, topography and vegetation, this relationship reversed: broadleaf‐associated communities emerged as positively associated with soil carbon stocks, alongside greater extramatrical hyphal proportion. Specific ectomycorrhizal fungi, notably Russula ochroleuca and Lactarius tabidus , also emerged as indicators of carbon‐rich soils, offering finer resolution than broad woodland type classifications. The reversal between single and multiple‐predictor models suggests that the relationship between conifer‐associated communities and soil carbon observed at broader scales may partially reflect correlated environmental conditions rather than direct fungal effects and that the independent contribution of ectomycorrhizal fungi to carbon storage may be stronger in broadleaf systems. Our findings demonstrate that ectomycorrhizal community composition predicts soil carbon storage at landscape scales, and suggest that integrating fungal data into ecological prediction frameworks can improve carbon monitoring and management. Read the free Plain Language Summary for this article on the Journal blog.