Long term atmospheric nitrogen (N) deposition affects carbon (C) and N cycling in forest ecosystems, thus changing the stable C isotope abundance of plants and soils. It is still unclear whether the input of extra nitrogen will affect the response of the δ13C and δ15N of dominant tree species and soil to long-term N addition in forest ecosystems, especially in a temperate secondary forest. We conducted a 14-year experiment of N addition to study the effects of long-term N addition on the δ13C and δ15N of Populus davidiana (PD) and Betula platyphylla (BP). The results showed that after 14 years of continuous N addition, the sample plot was still in the N limited state. Nitrogen addition significantly increased the 15N in PD and BP leaves, but high N addition did not enrich the 15N by increasing N loss in N rich sites. With the increase of soil depth, soil organic carbon (SOC) decreased significantly, and the soil 13C and 15N were enriched. PD and BP13C and C had high stability, and were not affected by N addition. PD was more “N deficient” than BP, while BP was more 15N rich than PD. Under the condition of N limitation, the addition of low and high N inhibited the turnover of organic C in PD which was beneficial to its C sequestration. However, high N treatment promoted the turnover of SOC. These results further deepen our understanding of the C and N cycle mechanism in temperate forests under the background of N deposition.