The effects of nitrogen (N) constraints on future terrestrial carbon (C) dynamics are investigated using the O-CN land surface model. The model's responses to elevated [CO2] and soil warming agree well with observations made in ecosystem manipulation studies. N dynamics reduce terrestrial C storage due to CO2 fertilization over the period 1860–2100 by ∼50% (342 Pg C) mainly in mid-high latitude ecosystems, compared to a simulation not accounting for N dynamics. Conversely, N dynamics reduce projected losses of land C due to increasing temperature by 16% (49 Pg C); however, this effect is prevalent only in mid-high latitude ecosystems. Despite synergistic interactions, the balance of these opposing effects is a significant reduction in future net land C storage. Terrestrial N dynamics thereby consistently increase atmospheric [CO2] in the year 2100 with a median value of 48 (41–55) ppmv, corresponding to an additional radiative forcing of 0.29 (0.28–0.34) W m−2.