Soil nitrogen (N) is an important limiting element for forest production, as it is one of the required nutrients for plant development in forest ecosystems. However, research on the dynamic changes of soil N in Chinese fir plantations with different stand ages remains elusive, especially from the perspective of aggregates. Therefore, this study aimed to investigate the changes in the contents and stocks of total nitrogen (TN) and labile nitrogen (LN) fractions in various soil aggregates (>2, 2-1, 1-0.25, and <0.25 mm) with different stand ages of Chinese fir plantations in Guangxi, China. Additionally, soil aggregates were categorized using the optimum moisture sieving method and compared soil with the TN and LN fractions of aggregates and bulk soil. The LN fractions included alkali-hydrolyzable nitrogen (AN), particulate organic nitrogen (PON), microbial biomass nitrogen (MBN), ammonium nitrogen (NH4 +-N), and nitrate nitrogen (NO3 --N). Results showed that stand ages and aggregate properties considerably affected soil N contents and stocks in Chinese fir plantations. Regardless of stand age, soil N contents increased as aggregate size dropped, although soil N stocks displayed the opposite tendency. Both soil N contents and stocks of Chinese fir plantations grew considerably until 17 years ago and then dropped dramatically thereafter. Pearson’s correlation analysis and redundancy analysis (RDA) further demonstrated that the key factors influencing soil N contents and stocks were soil aggregate stability and macro-aggregate proportions (>2 mm), which indicated that increasing coarse macro-aggregate proportions and soil aggregate stability was conducive to promoting the accumulation of soil N storage in Chinese fir plantations. In conclusion, our study reported the importance of maintaining the stability and composition of soil aggregate for N pool storage and circulation during the development of Chinese fir plantations, which offered new theoretical ideas for improving soil fertility and managing sustainably for Chinese fir plantations.