Moso bamboo expansion into subtropical coniferous forests reduces microbial carbon use efficiency and destabilizes soil organic carbon

Moso bamboo ( Phyllostachys edulis ) is a fast-growing industrial crop, but its ongoing expansion into native forests may compromise soil organic carbon (SOC) stability. To assess these effects, we quantified SOC fractions and microbial traits in pure Chinese fir (CF), bamboo–fir mixed stands (BCF), and pure bamboo (BF) forests in subtropical China. We found that SOC was significantly lower in BF than in CF, with mineral-associated organic carbon (MAOC) and microbial necromass carbon reduced by 47.94 % and 20.70 %, respectively. Microbial carbon use efficiency (CUE) also significantly declined in BF. Although BCF exhibited the highest total SOC and particulate organic carbon (POC) contents, the elevated POC/MAOC ratio indicated reduced SOC stability. Structural equation modeling revealed that increased soil ammonium suppressed microbial CUE, thereby reducing microbial necromass inputs, and ultimately MAOC formation. Bamboo expansion also enhanced β-glucosidase, peroxidase, and polyphenol oxidase (PPO) activities, accelerating MAOC decomposition. Furthermore, declines in fungal diversity under bamboo expansion likely weakened MAOC formation. In contrast, POC was positively associated with N-acetyl-β- D -glucosaminidase and PPO activities but was unrelated to microbial necromass or CUE. Overall, Moso bamboo expansion reduced MAOC content and weakened SOC stability by lowering microbial CUE and necromass production. This study provides mechanistic evidence that bamboo expansion, although economically valuable, may reduce SOC sustainability in subtropical forests, underscoring the need for management strategies that balance production benefits with long-term ecosystem functions. ● Bamboo expansion exerted a direct positive effect on POC content. ● Moso bamboo expansion significantly increased POC/MAOC ratio. ● Pure bamboo forest had significantly lower SOC and MAOC contents than Chinese fir forest. ● Bamboo expansion suppresses MAOC formation via NH 4 -mediated microbial CUE.