Rising atmospheric CO₂ concentrations may alter the nitrogen (N) content of ecosystems by changing N inputs and N losses, but responses vary in field experiments, possibly because multiple mechanisms are at play. We measured N fixation and N losses in a subtropical oak woodland exposed to 11 years of elevated atmospheric CO₂ concentrations. We also explored the role of herbivory, carbon limitation, and competition for light or nutrients in shaping the response of N fixation to elevated CO₂. Elevated CO₂ did not significantly alter gaseous N losses, but lower recovery and deeper distribution in the soil of a long-term ¹⁵N tracer indicated that elevated CO₂ increased leaching losses. Elevated CO₂ had no effect on nonsymbiotic N fixation, and had a transient effect on symbiotic N fixation by the dominant legume. Elevated CO₂ tended to reduce soil and plant concentrations of iron, molybdenum, phosphorus, and vanadium, nutrients essential for N fixation. Competition for nutrients and herbivory likely contributed to the declining response of N fixation to elevated CO₂. These results indicate that positive responses of N fixation to elevated CO₂ may be transient and that chronic exposure to elevated CO₂ can increase N leaching. Models that assume increased fixation or reduced N losses with elevated CO₂ may overestimate future N accumulation in the biosphere.