Moso bamboo (Phyllostachys edulis) expansion drives divergent mycorrhizal responses and stand reorganization via soil nitrogen and phosphorus imbalance and microbial network restructuring

• Moso bamboo differentially regulates mycorrhizae in L. chinense and S. japonica . • It simplified L. chinense ’s microbial network but enhanced complexity for S. japonica. • Our model shows bamboo affects plants via nutrients and microbes (63% indirect effect). Moso bamboo ( Phyllostachys edulis ) expansion into adjacent forests reduces plant diversity and disrupts soil nutrient cycling. However, the ecological mechanisms through which it drives stand simplification via soil-microbe-plant system interactions remain systematically unexplored. To unravel these mechanisms, we investigated different stand layers along an expansion gradient from pure moso bamboo forests to coniferous-broadleaved mixed forests from a soil-microbe-plant perspective. Results showed that moso bamboo expansion significantly altered soil nitrogen and phosphorus distributions, with a 56.8 % increase in ammonium nitrogen, a 36.3 % decrease in nitrate nitrogen, and a 46.3 % decrease in available phosphorus. These changes enhanced the competitive advantage of bamboo and Serissa japonica in the regeneration layer but suppressed Loropetalum chinense in the succession layer. Expansion reduced the mycorrhizal colonization rate of L. chinense by 24.04 %, accompanied by a decline in the abundance of mycorrhizal fungi such as Oidiodendron . Conversely, S. japonica exhibited a 9.9 % increase in mycorrhizal colonization, along with greater microbial network complexity and functionality. Partial least squares structural equation modeling indicated that expansion indirectly suppressed mycorrhizal colonization and abundance of L. chinense and promoted those of S. japonica by altering soil nitrogen-phosphorus balance and beneficial microbe abundance (indirect effect > 64.3 %), with mycorrhizal colonization being the key pathway ( β > 0.925). Overall, moso bamboo expansion drives stand restructuring by differentially affecting plant-mycorrhizal interactions via soil nutrient redistribution and microbial community reassembly. These findings provide a novel theoretical framework for managing bamboo expansion and promoting sustainable forest management.