The soil organic carbon (SOC) pool is the largest carbon pool in terrestrial ecosystems, and a small change in the pool will result in a great change in atmospheric CO₂ concentration. The input of labile organic carbon will accelerate or mitigate the mineralization of SOC through a positive or negative priming effect and eventually affect soil carbon balance. In the present study, we sampled forest soils at different restoration years from Changting County, Fujian Province, and added ¹³C-labeled glucose to those soils to investigate the impact of labile organic carbon input on the priming effect. The results showed that the direction and magnitude of the priming effect induced by the input of labile organic carbon were dependent on the restoration age. A positive priming effect was observed immediately after the application of labile organic carbon. However, a shift from positive priming effect to negative effect occurred in the 15- and 30-year-old Pinus massoniana forests as well as natural forest with time. Throughout the experiment (59 days), the input of labile organic carbon accelerated the mineralization of SOC in the bare soil and 5-year-old Pinus massoniana forest. The amount of SOC-derived CO₂ emissions increased by about 131% ±27% and 25% ±5%, respectively. However, the input of labile organic carbon declined by about 10% ±1% of SOC mineralization in the 15-year-old Pinus massoniana forest, and it had no significant effects on SOC mineralization in the 30-year-old Pinus massoniana forest and natural forest. The cumulative priming effect was positive correlated with the cumulative percent change in soil available N due to glucose addition during the 59 days of incubation. This indicates that the priming effect induced by the labile organic carbon input is governed by soil N availability, and microbes increased SOC mineralization to meet the N requirement.