Priority Colonization of Endophytic Fungal Strains Drives Litter Decomposition and Saprotroph Assembly via Functional Trait Selection in Karst Oak Forests

Litter decomposition dynamics are largely governed by microbial interactions. While the involvement of endophytic fungi in early-stage decomposition and microbial succession is well established, their species-specific contributions to decomposer community assembly remain insufficiently understood. This study investigated the effects of single-strain endophytic colonization using dominant species (<i>Tubakia dryina</i>, <i>Tubakia dryinoides</i>, <i>Guignardia</i> sp.) and rare species (<i>Neofusicoccum parvum</i>, <i>Penicillium citrinum</i>) on <i>Quercus acutissima</i> leaf decomposition through a controlled field experiment in a karst ecosystem. Endophytes accelerated decomposition rates across treatments but paradoxically reduced transient CO₂ emissions, linked to intensified microbial carbon and phosphorus limitations in late stages. Contrary to expectations, decomposition efficiency was governed by endophytic fungal species traits rather than colonization abundance, with rare species outperforming dominant taxa. Endophytes induced significant fungal community restructuring, reducing <i>Ascomycota</i> while enriching lignin-degrading <i>Basidiomycota</i>, but minimally affected bacterial composition. Co-occurrence networks revealed endophyte-driven fragmentation of microbial connectivity, with only two keystone fungal hubs (<i>Trechispora</i> sp. and <i>Russula carmesina</i>) identified compared to natural communities. Endophytic colonization improved fungal community assembly, mediated by an increase in lignin-degrading <i>Basidiomycota</i> and the suppression of pathogenic <i>Leotiomycetes</i> lineages. Our findings demonstrate that endophytes hierarchically regulate decomposer communities through phylogenetically conserved fungal interactions, prioritizing functional trait selection over competitive dominance, thereby stabilizing decomposition under nutrient constraints. This mechanistic framework advances predictions of litter decay dynamics in forest ecosystems undergoing microbial community perturbations.