In temperate forests, soils are the main sink for atmospheric N deposition. The main processes proposed for N retention are microbial and abiotic immobilization in soil organic matter. The relative importance of these processes as well as the kind of resulting chemical compounds are not totally understood. We carried out a laboratory incubation of Hg-sterilized and non-sterilized organic and organo-mineral soil horizons, labelled with either (NO3-)-N-15 or (NH4+)-N-15. The labelled samples were incubated for 1 hour, 1 day, or 6 days, then subjected to K2SO4 extraction and analysed with N-15 CPMAS NMR spectroscopy. N immobilization was already effective in all samples and treatments after 1 hour. The corresponding NMR spectra showed that part of the immobilized N-15 was already incorporated into an amide structure. In the sterilized soils labelled with (NH4+)-N-15, the tracer was rapidly and largely immobilized by an unknown process related to the presence of Hg. In the sterilized soils labelled with (NO3-)-N-15, between one-third and one-half of the added tracer was immobilized during the first hour and only 10% more over the 6 days. These results suggest that the sterilization was incomplete at first, allowing relatively great microbial immobilization during the first hour. By contrast, over a longer time, NO3- immobilization was significantly reduced to a level corresponding to an abiotic process as Hg sterilization became more effective. Even if the low signal-to-noise ratio precluded quantitative N-15 NMR measurements, we showed that the amide-peptide signal, considered as a biotic signature, was dominant in all cases.