The impact of atmospheric nitrogen (N) deposition on nitrous oxide (N<sub>2</sub>O) emissions in forest ecosystems is still unclear. Our study assessed the direct contribution of N deposition to N<sub>2</sub>O emissions in temperate forests exposed to chronic high N depositions using a <sup>15</sup>N labelling technique. In a Norway spruce stand (<i>Picea abies</i>) and in a beech stand (<i>Fagus sylvatica</i>) at the Solling, Germany, we used a low concentrated <sup>15</sup>N-labelled ammonium-nitrate solution to simulate N deposition. Nitrous oxide fluxes and <sup>15</sup>N isotope abundances in N<sub>2</sub>O were measured using the closed chamber method combined with <sup>15</sup>N isotope analyses. Emissions of N<sub>2</sub>O were higher in the beech stand (2.6 ± 0.6 kg N ha<sup>−1</sup> yr<sup>−1</sup>) than in the spruce stand (0.3 ± 0.1 kg N ha<sup>−1</sup> yr<sup>−1</sup>). We observed a direct effect of N input on <sup>15</sup>N-N<sub>2</sub>O emissions, which lasted for less than three weeks and was mainly caused by denitrification. No further increase in <sup>15</sup>N enrichment of N<sub>2</sub>O occurred during a one-year experiment, which was probably due to immobilisation of deposited N. The annual emission factor for N<sub>2</sub>O from deposited N was 0.1% for the spruce stand and 0.6% for the beech stand. Standard methods used in the literature applied to the same stands grossly overestimated emission factors with values of up to 25%. Only 6–13% of the total N<sub>2</sub>O emissions were derived from direct N depositions. Whether the remaining emissions resulted from accumulated anthropogenic N depositions or native soil N, could not be distinguished with the applied methods. The <sup>15</sup>N tracer technique is a useful tool, which may improve estimates of the current contribution of N deposition to N<sub>2</sub>O emissions.