Nitrogen(N) is an essential nutrient to plants. However, increasing N deposition into forest ecosystems due to will result in N saturation. The aims of this study were to (1) measure the composition of stable nitrogen isotope (δ¹⁵N) in fine roots in Cunninghamia lanceolata plantations at different ages and of various root orders, (2) examine whether the stable isotopes (δ¹⁵N) of fine roots in different orders were representative of the relative rates of soil net nitrogen (N) mineralization/nitrification across stands at different ages, and (3) determine whether fine root δ¹⁵N could be used as a better indicator of the rate of soil N cycling than foliar δ¹⁵N. We analyzed the δ¹⁵N of fine roots and leaves in C. lanceolata plantations at 3, 8, 14, 21, and 46 years of age in subtropical China and carried out a 28-day laboratory incubation to analyze soil potential net N mineralization and nitrification rates. The results showed that there were significant differences in the δ¹⁵N values of fine roots between different forest ages, the δ¹⁵N was higher in 3- and 46-year-old plantations than those at other ages. While there were no significant differences between δ¹⁵N values of fine roots in different root orders, the δ¹⁵N values of fine roots tended to be lower with the increased orders of root. The δ¹⁵N values of fine roots at all ages were significantly correlated with the potential net N mineralization and net nitrification rates, and the correlation coefficients between these parameters seemed to decrease with increasing root orders; however, this correlation was not significant between foliar δ¹⁵N at different ages and the soil N cycling rates. In conclusion, the result imply that fine root δ¹⁵N could be a better indicator of the relative rate of N cycling than the δ¹⁵N value of leaves, and first order roots were the best indicator. Moreover, fine root δ¹⁵N values suggested that N may still be a factor limiting the growth of the C. lanceolata plantations at the rapid-growth stages, despite the observation that subtropical China has the highest rates of N deposition in China.