Long-term field observations showed that N<sub>2</sub>O fluxes observed shortly after N application were not significantly affected by elevated CO<sub>2</sub> in the Giessen Free Air Carbon dioxide Enrichment (FACE) study. To further investigate this unexpected result a <sup>15</sup>N tracer study was carried out under controlled conditions where in parallel treatments either the NH<sub>4</sub><sup>+</sup> pool (<sup>15</sup>NH<sub>4</sub>NO<sub>3</sub>) or the NO<sub>3</sub><sup>−</sup> pool (NH<sub>4</sub><sup>15</sup>NO<sub>3</sub>) was enriched with <sup>15</sup>N. Fluxes of CO<sub>2</sub>, CH<sub>4</sub>, and N<sub>2</sub>O as well as the <sup>15</sup>N enrichment of the N<sub>2</sub>O were measured. Denitrifying Enzyme Activity (DEA), total denitrification (N<sub>2</sub> + N<sub>2</sub>O) and N<sub>2</sub>-to-N<sub>2</sub>O ratios were quantified in separate experiments. Over the 57 day incubation, N<sub>2</sub>O fluxes averaged 0.090 ng N<sub>2</sub>O-N g<sup>−1</sup> h<sup>−1</sup> under ambient and 0.083 ng N<sub>2</sub>O-N g<sup>−1</sup> h<sup>−1</sup> under elevated CO<sub>2</sub> (not significantly different). The N<sub>2</sub>O production processes were identified by a two-source model. Results showed that N<sub>2</sub>O must have also been produced by a third source – possibly related to organic N transformation – which was stimulated by elevated CO<sub>2</sub>. Soil CO<sub>2</sub> fluxes were approximately 20 % higher under elevated CO<sub>2</sub> than soil from ambient but the differences were not significant. CH<sub>4</sub> oxidation rates were on average −1.75 ng CH<sub>4</sub>-C g<sup>−1</sup> h<sup>−1</sup> in the elevated and −1.17 ng CH<sub>4</sub>-C g<sup>−1</sup> h<sup>−1</sup> in the ambient indicating that elevated CO<sub>2</sub> increased the CH<sub>4</sub> oxidation by 49 % compared to ambient CO<sub>2</sub> under controlled conditions. N fertilization increased CH<sub>4</sub> oxidation by 3-fold in both CO<sub>2</sub> treatments. CO<sub>2</sub> did not have any significant effect on DEA while total denitrification and N<sub>2</sub>-to-N<sub>2</sub>O ratios increased by 36 and 33 %, respectively. The results indicate that shortly after N application elevated CO<sub>2</sub> must have stimulated both the N<sub>2</sub>O production and reduction to N<sub>2</sub> to explain the increased N<sub>2</sub>-to-N<sub>2</sub>O ratio and at the same time explain the non-responsiveness of the N<sub>2</sub>O emissions. Thus, the observed variation of the CO<sub>2</sub> effect on N<sub>2</sub>O emissions throughout the year is possibly governed by the dynamics of the N<sub>2</sub>O reductase activity.