1 Swiss Federal Institute of Technology (EPFL), CH-1015 Lausanne, Switzerland
2 Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), CH-8903 Birmensdorf, Switzerland
3 Paul Scherrer Institut (PSI), CH-5232 Villigen-PSI, 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 15N-labelled soil samples collected one week, three months and one year following a single in situ application of either 15NO3- or 15NH4+ in two montane forests ecosystems in Switzerland: Grandvillard (beech forest on a calcareous, well-drained soil, 650 m a.s.l.) and Alptal (spruce forest on hydromorphic soil, 1200 m a.s.l.). After 15NH4+ application, recovery rates in the soil were lower in Alptal than in Grandvillard due to a high absorption by mosses. At both sites, the organic soil layers retained most of the tracers at all three sampling times between one week and one year. In Grandvillard, the hydrolysability (hydrolysable N / total N) of 15N was on average 79% and thus similar to the hydrolysability of native N. This similarity is probably due to the rapid incorporation of N into organic molecules, followed by stabilisation of the recalcitrant N pool through organo-mineral bonds with soil minerals. In Alptal, the 15N hydrolysability was higher than that of native N, particularly after 15NH4+ application (15N: 84%; native N: 72%). On both sites, 15N and native N hydrolysability remained constant between one week and one 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.
Keywords: acid hydrolysis, forest soil, 15N tracer methods, N immobilization