Abstract: The rising atmospheric CO₂ concentration significantly changed soil nitrogen (N) cycling which is important for us to predict the carbon (C) sequestration potential of terrestrial ecosystems. The natural abundance of N isotope as an integrative indicator of ecosystem N cycling processes can effectively indicate the effect of elevated CO₂ on soil N cycling processes. Here, we used an open top chamber experiment to examine the effects of elevated CO₂for ten years on the natural abundance of Quercus mongolica, soil and microbial biomass C and N isotopes in northeastern China. Our results showed that elevated CO₂ significantly changed soil N cycling processes, resulting in the increase of microbial and leaf δ¹⁵N; stimulated the decomposition of ¹³C-enriched soil organic C, and offset the effect of more ¹³C-depleted plant photosynthetic C inputs, resulting in unchanged δ¹³C of soil dissolved organic C and microbes under elevated CO₂. These results indicated that elevated CO₂ likely increased the mineralization of soil organic matter, and the system is getting more N-limited.