Afforestation is acknowledged as a key strategy for increasing carbon (C) and nitrogen (N) sequestration. However, the temporal dynamics and environmental drivers of organic carbon (OC) and N turnover within soil aggregates during afforestation remain poorly understood. Here, we applied a delta 13C- and delta 15N-based twoendmember isotope mixing model to quantify the proportions, stocks, and decadal average accumulation rates (AARs) of newly derived and legacy OC and N in soil aggregates along a 30-year afforestation chronosequence on China's Loess Plateau, spanning gradients of soil texture and climate. Afforestation substantially altered aggregate-level C and N dynamics, with stocks of newly derived OC and N progressively increasing, whereas legacy pools remained largely stable. The first decade represented a critical window of biogeochemical transformation, during which surface (0-10 cm) macroaggregates (MAs) acted as hotspots for new OC and N accumulation, exhibiting the highest AARs that declined sharply in subsequent decades. Environmental factors (MAP, MAT, pH) strongly controlled early-stage OC and N accumulation, but their influence weakened substantially over time, indicating a gradual shift from climate-driven dynamics to intrinsic soil stabilization processes. Overall, this study reveals the differentiated dynamics of new and legacy OC and N accumulation during afforestation and their environmental controls, underscoring the critical role of early-stage processes in aggregate-level C and N sequestration. The transition from strong early environmental controls to later intrinsic stabilization highlights the necessity of incorporating stage- and depth-specific representations into terrestrial C-N cycling models to better capture the mechanisms underpinning long-term C storage.