The water-sediment interface acts as a core reactor for the riverine carbon cycle, releasing substantial CO2 into the atmosphere through the microbial mineralization of dissolved organic carbon (DOC). However, the role of riverbed sediment in regulating organic carbon mineralization and its quantitative mechanisms remain unclear, particularly in ecosystems undergoing large-scale vegetation restoration. This study investigated rivers within a revegetated small watershed on the Loess Plateau. Using laboratory dark incubation experiments at constant temperature, combined with analyses of microbial communities and functional genes, we elucidated the seasonal mechanisms governing the sediment's contribution to organic carbon mineralization. Results indicated that riverbed sediment served as a continuous carbon source for the overlying water; after a 42-day incubation, particulate organic carbon (POC) content was at 0.598-0.824 times its initial value. The sediment significantly enhanced total CO2 emission primarily by alleviating nitrogen limitation in the water column and altering DOC composition. The contribution of sediment to organic carbon mineralization potential (CS) peaked during the wet season (52.020 +/- 8.054%), coinciding with the lowest cumulative mineralization, and reached a minimum during the dry season (43.417 +/- 6.688%), when cumulative mineralization was highest. A high CS state was characterized by a low abundance of carbon degradation (e.g., xylA, sga) and carbon fixation (e.g., rbcL) genes in the initial water column, and was significantly positively correlated with sediment fungi capable of decomposing recalcitrant organic matter (e.g., Amniculicola, Castanediella). These findings provide a theoretical reference for understanding the drivers of microbial spatiotemporal patterns in riverine carbon cycling and offer a scientific basis for comprehensively assessing the ecological benefits of vegetation restoration on the Loess Plateau.