Mixed mycorrhizal strategies and environmental pressures synergistically regulate diversity-soil total organic carbon relationships in temperate forests

• First mixed mycorrhizal index quantifies AM-EM synergistic effects on forest carbon. • Mixed mycorrhizal communities boost soil organic carbon by 19.2% via diversity. • Human disturbance strengthens diversity-carbon links; drought effects insignificant. • Mycorrhizal strategies regulate carbon storage through direct and indirect pathways. • Climate-adaptive forest management strategy based on mycorrhizal complementarity. Understanding how mycorrhizal strategies mediate forest diversity-carbon storage relationships under environmental stress remains a critical knowledge gap for predicting ecosystem responses to global change. While previous studies have focused on single mycorrhizal types, the synergistic effects of mixed arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) strategies on diversity-carbon dynamics under multiple stressors remain poorly understood. We analyzed 404 forest plots across northeastern China to examine how mixed mycorrhizal strategies influence soil total organic carbon (TOC) through taxonomic diversity (TD) and functional diversity (FD) pathways under varying human disturbance and drought stress. We employed structural equation modeling to quantify direct and indirect effects, complemented by random forest analysis to assess variable importance. Mixed mycorrhizal communities exhibited significantly higher TOC (3.42 ± 1.21 %) than single mycorrhizal communities (2.87 ± 0.95 %, p < 0.001). Structural equation modeling revealed that mixed mycorrhizal strategies enhanced TOC through both direct pathways (β = 0.1381, p < 0.001) and indirect pathways via taxonomic diversity (β = 0.0413) and functional diversity (β = 0.0450). Total effects analysis showed mixed mycorrhizal strategies contributed 22.4 % to TOC variation, with human disturbance significantly strengthening diversity-TOC relationships (p < 0.001), while drought effects were non-significant. This study provides the first large-scale evidence that mixed mycorrhizal strategies serve as fundamental mechanisms linking forest biodiversity to carbon storage, offering practical guidelines for climate-adaptive forest management that simultaneously enhances carbon sequestration and biodiversity conservation under increasing anthropogenic pressures.