Increased N fixation in response to rising atmospheric CO2 could support carbon uptake in the terrestrial biosphere, because plant growth is often limited by N. While elevated CO2 often enhances N fixation in laboratory conditions, does this also consistently happen in the field, without added fertilizer, where soils are undisturbed, and in intact plant communities? I reviewed studies of N fixation responses to elevated CO2, searching the literature from 1930 to the present. Overall, elevated CO2 tends to increase N fixation, but the effect is smaller in the field, with intact plant communities, without added fertilizer, and with undisturbed soil, compared to less artificial conditions. While elevated CO2 has the potential to increase nitrogen inputs to ecosystems, it appears that constraints through competition and nutrient availability limit responses in native ecosystems. Future projections of carbon uptake by the terrestrial biosphere should take into account not only the N dependence of C accumulation, but also the capacity for ecosystems to supply the N required.