Ectomycorrhizal fungi redirect soil carbon stabilization pathways under nitrogen enrichment by regulating microbial necromass formation

Atmospheric nitrogen (N) deposition is altering forest soil carbon dynamics, yet the role of ectomycorrhizal (ECM) symbiosis in regulating soil organic carbon (SOC) stabilization under elevated N remains poorly understood. We investigated how N addition (0, 5, 10 and 20 mM urea) and ECM colonization interact to influence SOC partitioning, microbial functioning, and stabilization mechanisms in Eucalyptus grandis × Eucalyptus urophylla soils. ECM colonization remained consistently high across the N gradient, but fungal biomass declined with increasing N, indicating shifts in fungal allocation strategies. N addition alone favored accumulation of transient particulate organic carbon, whereas ECM inoculation promoted mineral-associated organic carbon (MAOC) at intermediate N levels, despite reductions in SOC hydrophobicity and chemical recalcitrance, suggesting that ECM-driven stabilization operates independently of chemical persistence. ECM fungi constrained microbial oxidative enzyme investment, notably suppressing polyphenol oxidase, thereby reshaping microbial carbon processing. Plant-derived CAZymes dominated SOC depolymerization, while SOC partitioning was independently regulated by a Bradyrhizobium–Penicillium-centered microbial guild and by the CAZy enzyme families GH104 (bacterial) and GH17 (fungal). MAOC accumulation was more closely associated with microbial necromass formation and microbial carbon pump efficiency than with lignin chemistry. Collectively, these findings demonstrate that ECM symbiosis mediates soil carbon fate under N enrichment by coordinating microbial community composition, enzymatic activity, and necromass pathways, redirecting carbon from rapid mineralization to biologically mediated stabilization. This study highlights ECM fungi as critical biotic controllers of SOC persistence in N-enriched plantation ecosystems.