Moso bamboo (Phyllostachys edulis (Carrie`re) J. Houz.) exhibits robust cloning and propagation capabilities, enabling its expansion into adjacent forest communities. However, previous studies on Moso bamboo expansion have predominantly focused on virgin forests, and studies on its effects on planted forests, particularly adjacent coniferous forests, remain limited. Therefore, this study investigated the effect of Moso bamboo expansion into planted forests of Masson pine (Pinus massoniana Lamb.) on soil metabolites. The study was conducted using untargeted metabolomics under different slope directions and positions in a typical karst river basin, and the relationships with soil metabolites and environmental factors were delineated. Moso bamboo expansion primarily affected glycerophospholipid metabolism and flavonoid biosynthesis in the soil. The alpha diversity of soil metabolites increased with the slope position (p < 0.05). However, no significant difference in soil metabolites was observed between different slopes. The effects of soil organic matter, total phosphorus, and soil microbial community composition on soil metabolites were significant (p < 0.05). Structural equation modeling suggested the existence of these two potential pathways of influence; the slope position and direction jointly affected soil metabolism through soil properties including pH (37.6 %), OM (49.3 %), Ca (14.4 %), HN (10.1 %), and TP (10.4 %), and microbial communities by Observed Species (19.3 %), Chao1 (12.3 %), ACE (12.1 %), and Shannon indices (20.3 %). Furthermore, the influence of slope positions and directions directly affected soil properties, subsequently affecting soil metabolism through the mediation of soil microbial communities by Observed Species (70.6 %), Chao1 (59.9 %), ACE (57.9 %), and Shannon indices (52.5 %). Therefore, we propose that assessing the rapid response on soil metabolites under varying slope positions and directions, along with their connection to soil environmental indicators, may provide insight into the ecological implications of Moso bamboo expansion. Our findings provide valuable insights for understanding how Moso bamboo cover dynamics affect soil metabolites in a typical karst river basin.
