Mechanisms Underlying Rhizosphere Nutrient Enrichment Driven by Flowering-Mediated Metabolite–Microbe Interactions in the Monocarpic Bamboo Chimonobambusa opienensis

Background: The monocarpic woody bamboo Chimonobambusa opienensis flowers synchronously and undergoes post-flowering senescence, imposing high nutritional demands during reproduction. Field investigations have documented increases in rhizosphere nutrients during bamboo blooming, but the underlying metabolic–microbial mechanisms remain unclear. Methods: Eight biological replicates of flowering (FD) and non-flowering (NF) C. opienensis across two locations were used to investigate differences in rhizosphere metabolites, microbiota, functional genes, and soil nutrient stoichiometry. A total of 2196 metabolites were identified, including 108 specific to FD and 63 specific to NF. Flowering was associated with metabolic reprogramming: flavonoids (2′,5,6-trimethoxyflavone) and antioxidant metabolites increased, whereas growth-promoting phytohormones and structural maintenance metabolites decreased. This metabolic transition was accompanied by substantial functional reorganization of the rhizosphere microbiome. Ammonia-oxidizing archaea (AOA) and nitrite-oxidizing bacteria (NOB), functional guilds associated with nitrification, were selectively enriched in FD rhizospheres, suggesting a shift from carbon-cycle predominance toward nitrogen-activation potential. NF rhizospheres harbored a microbial consortium associated with decomposition and methanogenesis of complex organic matter. In FD rhizospheres, nitrogen and phosphorus contents were significantly higher (p < 0.05). Correlation analyses revealed strong associations among root metabolites, functional microbial guilds, and soil nutrient pools. Conclusions: The rhizosphere metabolite–microbe–nutrient axis is associated with the increased nutrient demands of flowering and may support reproductive success in monocarpic bamboos. A putative “metabolic signals–microbial functions–nutrient supply” interaction network is identified in the flowering rhizosphere, advancing our understanding of perennial clonal plant–microbe interactions during reproductive transitions.