Increased anthropogenic nitrogen (N) inputs can alter the N cycle and affect forest ecosystem functions. The impact of increased N deposition depends among others on the ultimate fate of N in plant and soil N pools. Short-term studies (3–18 months) have shown that the organic soil layer was the dominant sink for N. However, longer time scales are needed to investigate the long-term fate of N. Therefore, the soils of four experimental forest sites across Europe were re-sampled ~ 2 decades after labelling with ¹⁵N. The sites covered a wide range of ambient N deposition varying from 13 to 58 kg N ha⁻¹ year⁻¹. To investigate the effects of different N loads on ¹⁵N recovery, ambient N levels were experimentally increased or decreased. We hypothesized that: (1) the mineral soil would become the dominant ¹⁵N sink after 2 decades, (2) long-term increased N deposition would lead to lower ¹⁵N recovery levels in the soil and (3) variables related to C dynamics would have the largest impact on ¹⁵N recovery in the soil. The results show that large amounts of the added ¹⁵N remain in the soil after 2 decades and at 2 out of 4 sites the ¹⁵N recovery levels are higher in the mineral soil than in the organic soil. The results show no clear responses of the isotopic signature to the changes in N deposition. Several environmental drivers are identified as controlling factors for long-term ¹⁵N recovery. Most drivers that significantly contribute to ¹⁵N recovery are strongly related to the soil organic matter (SOM) content. These findings are consistent with the idea that much of the added ¹⁵N is immobilized in the SOM. In the organic soil layer, we identify C stock, thickness of the organic layer, N-status and mean annual temperature of the forest sites as most important controlling factors. In the mineral soil we identify C stock, C content, pH, moisture content, bulk density, temperature, precipitation and forest stand age as most important controlling factors. Overall, our results show that these temperate forests are capable of retaining long-term increased N inputs preferably when SOM availability is high and SOM turnover and N availability are low.