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
Atmospheric N deposition was shown to be mainly retained in the soil of temperate forest ecosystems. The soil organic layers are the main sink for deposited N but the mechanisms and the organic fractions involved are still poorly defined. We therefore performed a hot acid hydrolysis on 15N labelled soil samples collected within a one-year field labelling experiment (a single application of 15NO3- or 15NH4+) and carried out in two contrasting forests ecosystems (Grandvillard and Alptal). For 15NH4+ application, recovery rates in the soil were found lower in Alptal than in Grandvillard, due to a fast absorption by the mosses. The both sites organic layers retained most of the tracers within one year course. In Grandvillard, the hydrolysability (hydrolysable N / total N) of 15N reached an annual average of 79% and was similar to the hydrolysability of native N. This similarity could be determined by a rapid N stabilisation into the recalcitrant N pool through organo-mineral bounds. In Alptal 15N hydrolysability was higher than that of native N, particularly in case of 15NH4+ application (15N: 84% ; native N: 72%) and could be explained by a rapid microbial turnover of hydrolysable N. On both sites, 15N and native N hydrolysability was constant over the year. We propose that deposited N immobilisation within the recalcitrant pool is effective in the long term. Furthermore, the biological recycling must be involved in the stability of the hydrolysable N fraction.