N-15 immobilization in forest soil: a sterilization experiment coupled with N-15 CPMAS NMR spectroscopy

Abstract

In temperate forests, soils are the main sink for atmospheric N deposition. The main processes proposed for N retention are the microbial immobilization and the abiotic fixation in soil organic matter. The relative importance of such processes as well as the kind of resulting chemical compounds are not totally resolved. In order to improve our understanding on the subject, we carried out a laboratory incubation of sterilized and unsterilized soils (organic and organo-mineral), labeled with 15NO3 - or 15NH4 +, which are the main form of N deposition. Soils were incubated during one hour, one day and one week times and then subjected to a K2SO4 extraction and to 15N CPMAS spectroscopy measurements. After one hour of incubation, immobilization was already effective within all the incubated soils. The corresponding NMR spectra were difficult to interpret since the signal-to-noise ratio was low. However, part of the immobilized 15N was already incorporated as amides. In the sterilized soils labeled with 15NH4 +, a chemical process connected with the presence of Hg, immobilized the tracer rapidly and massively (between 80% to 90% were unextractable after one hour in the organic sterilized soils against 50% in the unsterilized). However, no corresponding specific peak was observable on the NMR spectra. In the sterilized soils labeled with 15NO3 -, between one half and one third of the added tracer was immobilized during the first hour and then, in the organic layer, 10% more during the week. We suppose, on the one hand, that the incomplete sterilization in the very short term explains the one-hour immobilization; on the other hand, an abiotic process seems to be responsible for the NO3 - immobilization over the week. In the unsterilized soils, approximately 50% of 15N was immobilized in the OL-A samples (approximately 40% in the Aca layer) during the first hour and approximately 80% during the week (approximately 60% in the Aca layer). The dynamics of immobilization were very similar for 15NH4 + and 15NO3 - , mainly immobilized as amides. Within the framework of our study and because of the low signal-to-noise ratio obtained by 15N NMR measurements, the rate of 15N immobilized as amide could not be quantified. However, we showed that the amides-peptides signal was dominant whichever layer is concerned, or chemical form added or even whether the soil is sterilized or not. Consequently, we are able to confirm the importance of the proteinaceous compounds for the immobilization of N in the soil.