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Afforestation of former agricultural land is a means to mitigate anthropogenic greenhouse gas emissions. The objectives of this study were (1) to assess the effect of oak (<I>Quercus robur</I>) and Norway spruce (<I>Picea abies [L.] Karst.</I>) stands of different stand ages (13–17 and 40 years after afforestation, respectively) on N₂O and CH₄ exchange from the soil under these species and (2) identify the environmental factors responsible for the differences in gas exchange between tree species of different ages. N₂O and CH₄ fluxes (mean ± SE) were measured for two years at an afforested site. No species difference was documented for N₂O emission (oak: 4.2 ± 0.7 μg N₂O-N m⁻² h⁻¹, spruce: 4.0 ± 1 μg N₂O-N m⁻² h⁻¹) but the youngest stands (1.9 ± 0.3 μg N₂O-N m⁻² h⁻¹) emitted significantly less N₂O than older stands (6.3 ± 1.2 μg N₂O-N m⁻² h⁻¹). CH₄ exchange did not differ significantly between tree species (oak: −8.9 ± 0.9, spruce: −7.7 ± 1) or stand age (young: −7.3 ± 0.9 μg CH₄-C m⁻² h⁻¹, old: −9.4 ± 1 μg CH₄-C m⁻² h⁻¹) but interacted significantly; CH₄ oxidation in the soil increased with stand age in oak and decreased with age for soils under Norway spruce. We conclude that the exchange of N₂O and CH₄ from the forest soil undergoes a quick and significant transition in the first four decades after planting in both oak and Norway spruce. These changes are related to (1) increased soil N availability over time as a result of less demand for N by trees in turn facilitating higher N₂O production in older stands and (2) decreasing bulk density and increased gas diffusivity in the top soil over time facilitating better exchange of N₂O and CH₄ with the atmosphere.