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
4 Technische Universität München, D-85350 Freising, Germany
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-sterilised and non- sterilised organic and organo- mineral soil horizons, labelled with either 15NO3- or 15NH4+. The labelled samples were incubated for one hour, one day, or six days, then subjected to K2SO4 extraction and analysed with 15N CPMAS NMR spectroscopy. N immobilization was already effective in all samples and treatments after one hour. The corresponding NMR spectra showed that part of the immobilized 15N was already incorporated into an amide structure. In the sterilised soils labelled with 15NH4+, the tracer was rapidly and largely immobilized by an unknown process related to the presence of Hg. In the sterilised soils labelled with 15NO3-, between one third and one half of the added tracer was immobilized during the first hour and only 10% more over the six days. These results suggest that the sterilisation 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 sterilisation became more effective. Even if the low signal-to-noise ratio precluded quantitative 15N NMR measurements, we showed that the amide-peptide signal, considered as a biotic signature, was dominant in all cases.