Nitrogen (N) from atmospheric deposition has been shown to be mainly retained in the organic soil layers of temperate forest ecosystems, but the mechanisms and the physico-chemical fractions involved are still poorly defined. We performed a hot-acid hydrolysis on N-15-labelled soil samples collected 1 week, 3 months and 1 year following a single in situ application of either (NO3-)-N-15 or (NH4+)-N-15 in two montane forest ecosystems in Switzerland: Grandvillard (beech forest on a calcareous, well-drained soil, 650 m above sea level) and Alptal (spruce forest on hydromorphic soil, 1200 m above sea level). After (NH4+)-N-15 application, recovery rates in the soil were smaller in Alptal than in Grandvillard through a large rate of absorption by mosses. At both sites, the organic soil layers retained most of the tracers at all three sampling times between 1 week and 1 year. In Grandvillard, the hydrolysable fraction (hydrolysable N : total N) of N-15 was on average 79% and thus similar to the hydrolysable fraction of native N. This similarity is probably because of the rapid incorporation of N into organic molecules, followed by stabilization of the recalcitrant N pool through organo-mineral bonds with soil minerals. In Alptal, the N-15 hydrolysable fraction was greater than that of native N, particularly after (NH4+)-N-15 application (N-15, 84%; native N, 72%). At both sites, N-15 and the fraction of hydrolysable native N remained constant between 1 week and 1 year. This shows that both the recalcitrant and the hydrolysable pools are stable in the mid-to long-term. We present arguments indicating that biological recycling through microbes and plants contributes to the stability of the hydrolysable N fraction.