Root exudates drive stage-dependent soil aggregate stability through direct and microbial pathways

Root exudates are key mediators of plant-microbe-soil interactions, yet how their temporal dynamics impact soil aggregate stability remains poorly understood. We investigated four dominant riparian species in the drawdown zone of the Three Gorges Reservoir to examine how root exudate flux and stoichiometric characteristics regulate rhizosphere microbial communities and soil aggregation at the middle and late growth stages. Exudate composition significantly influenced microbial diversity, with bacterial communities being more sensitive to DON and C:N ratio, whereas fungal communities responded to DOC, DON, and C:N. Soil aggregate stability exhibited pronounced stage-dependent dynamics, with stronger exudate-related effects at the late growth stage. Partial least squares path modeling (PLS-PM) showed that root exudates affected soil aggregate stability through both direct and microbially mediated pathways. The direct pathway was dominant, especially at the late stage, when the model explained 67% of the variance in aggregate stability. Microbial pathways provided additional stage-dependent contributions, with the fungal-mediated indirect effect shifting from negative at the middle stage to positive at the late stage, while the bacterial pathway remained relatively weak. These findings reveal a lagged plant-microbe-soil feedback in which root exudates first reshape microbial community structure and biochemical processes, with soil structural benefits becoming more evident as plant growth progresses. This study provides new insight into rhizosphere regulation of soil aggregation and supports vegetation restoration strategies for hydrologically fluctuating riparian ecosystems.