Microbial mechanisms underlying active organic carbon pool increases via rhizodeposition by Moso bamboo under in situ crown 13CO2 enrichment

Plant rhizodeposition strongly mediates plant-soil-microbe interactions and impacts the soil active organic carbon (C) pool. However, the mechanisms by which rhizodeposition affects the modulation of soil active organic C pool remain poorly understood. In this study, we investigated the effects of rhizodeposition on active organic C pool and microbial communities in Moso bamboo (Phyllostachys edulis), a clonal plant with rhizome-connected mother and offspring ramets. The crown of the mother ramet was enriched with ¹³CO₂ to trace rhizodeposition and assess changes in soil organic C pool and the microbial community across different root systems (culm roots and rhizome roots) during key growth stages (early, peak, branching, and leafing). Our results showed that crown CO₂ enrichment increased δ¹³C and the contents of microbial biomass C, dissolved organic C, and labile organic C (by 22.08 % to 43.67 %) in the rhizosphere, which significantly altered the bacterial community composition. Under crown CO₂ enrichment, the abundance of bacterial taxa and genes associated with recalcitrant C decomposition decreased by 29.08 %–77.38 % and 30.75 %–56.85 %, respectively; while taxa and genes related to CO₂ fixation increased by 51.22 %–267.00 % and 92.52 %–331.46 %, respectively. These microbial shifts contributed to an increase in the soil active organic C pool. In addition, the growth stage, rather than the root system, had a greater influence on rhizodeposition, with higher δ¹³C observed during the early and leafing stages compared to the peak and branching stages. In conclusion, our findings suggest that increases in the active organic C pool were primarily driven by enhanced C rhizodeposition, the stimulation of microbes involved in CO₂ fixation, and the suppression of bacteria involved in recalcitrant C decomposition, while being highly dependent on the growth stage. This study provides valuable insights into the interactions between plant rhizodeposition, microbial functions, and soil active organic C dynamics.