Nitrogen is an important factor limiting the productivity of terrestrial ecosystems. We used the method of spatiotemporal substitution and Pinus massoniana woodlands with different restoration history lengths as the research subject. In order to understand the effects of Dicranopteris dichotoma on nitrogen fixation during the vegetation restoration process, we measured the stable isotope ¹⁵N, the nitrogen content of different components, and the ratios of different components of nitrogen to total nitrogen in the topsoil between D. dichotoma soil (WD) and non-D. dichotoma soil (ND). The results showed that Dicranopteris dichotoma coverage increased the total nitrogen (TN) content of the topsoil in all Pinus massoniana woodlands. The δ¹⁵N values of WD were significantly decreased, from 33.8% to 83.1%, compared with those of ND (P < 0.05). With the increase of restoration time, the δ¹⁵N value of WD significantly decreased, while the δ¹⁵N value of ND did not significantly change (P > 0.05). The average contents of microbial biomass nitrogen (MBN), dissolved organic nitrogen (DON), and ammonium nitrogen (NH₄⁺-N) of WD were significantly higher than those of the ND under different restoration years (P < 0.05), but the content of nitrate nitrogen (NO₃^--N) had the complete opposite trend (P < 0.05). With the increase in the duration of restoration, the contents of MBN, DON, and NH₄⁺ in topsoil increased, while NO₃^--N content decreased. The ratios of different components of nitrogen to total nitrogen were in the following order:MBN > NH₄⁺-N > DON > NO₃^--N. Correlation analysis showed that the δ¹⁵N values of topsoil had highly significant positive correlations with the content of NO₃^--N, and highly significant negative correlations with the others (P < 0.01). Thus, compared with ND, Dicranopteris dichotoma coverage was helpful in improving the contents of TN, MBN, DON, and NH₄⁺-N in topsoil and reducing the risk of nitrate leaching loss during the vegetation restoration process, and promoted the retention and accumulation of soil nitrogen, which was beneficial to the restoration of the degraded red soil ecosystem.