Nitrogen inputs and losses in response to chronic CO 2 exposure in a subtropical oak woodland
Rising atmospheric CO 2 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 CO 2 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 CO 2. Elevated CO 2 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 CO 2 increased leaching losses. Elevated CO 2 had no effect on nonsymbiotic N fixation, and had a transient effect on symbiotic N fixation by the dominant legume. Elevated CO 2 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 CO 2. These results indicate that positive responses of N fixation to elevated CO 2 may be transient and that chronic exposure to elevated CO 2 can increase N leaching. Models that assume increased fixation or reduced N losses with elevated CO 2 may overestimate future N accumulation in the biosphere.