Retention and hydrolysable fraction of atmospherically deposited nitrogen in two contrasting forest soils in Switzerland

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.