Abstract: Microbial carbon use efficiency (CUE), a critical determinant of soil carbon cycling, exhibits spatiotemporal variations influenced by biotic and abiotic factors; however, interplay between forest type, seasonality, and fungal community dynamics remains poorly understood. Here, we investigated CUE patterns, soil enzyme activities, and fungal functional guilds across coniferous (Picea asperata, Larix gmelinii) and broadleaved (Betula albosinensis, Quercus aquifolioides) forests in subalpine China during early-, mid-, and late-growing seasons. The CUE of coniferous forests exhibited pronounced seasonal declines (45% reduction in September vs. May–July), whereas broadleaved forests maintained stable. Forest type caused small changes in CUE across seasons, with broadleaved was higher than coniferous forests in late-growing season. Soil fungal community composition diverged significantly between forest types: symbiotroph (mainly ectomycorrhizal fungi) dominated in coniferous soils (58%–81% relative abundance), correlating negatively with CUE, while saprotroph prevailed in broadleaved forests (21%–43%), showing positive CUE associations. Co-occurrence network in broadleaved forests had higher modularity and connectivity than coniferous forests, indicating greater resistance to seasonal variations. Structural equation modelling identified fungal guilds, soil C:N ratio, enzyme investment as primary drivers of CUE variation, with fungal composition mediating 28% CUE variability. Our findings highlight the critical role of fungal functional traits in modulating microbial metabolic efficiency and provide mechanistic insights for predicting soil carbon dynamics in forest ecosystems.

Key words: seasonal dynamic, symbiotic fungi, saprophytic fungi, soil enzyme activities, microbial biomass