Retention of atmospherically deposited nitrogen in soil : field and laboratory experiments using 15N isotope and 15N CPMAS NMR spectroscopy

Since a few decades, the balance of the nitrogen (N) cycle has been deeply disturbed by human activies. The global impact of these activities on the N cycle can be described as a doubling of the transfer from the vast and unreactive atmospheric pool to biologically available forms (N fixation). The main sources responsible for the increase of reactive N emissions are the use of artificial fertilisers (NH3) as well as the combustion processes (NOx). Reactive N is then transformed, transported by both the atmosphere and the hydrosphere and, finally, deposited on both the terrestrial and the aquatic ecosystems with potentially strong impacts. Amongst the terrestrial ecosystems, the temperate forests are particularly sensitive to reactive N increases for various reasons: they are located near to strongly anthropised areas and are thus subjected to strong depositions; they are naturally N-limited and their biochemical cycle can indeed be strongly influenced by additional N, which leads to eutrophication and potential impacts such as soil acidification, nitrate leaching and losses of biodiversity within microorganisms, plants and fauna communities. Previous studies carried out in temperate forest ecosystems have shown that the soil, namely the soil organic matter, acts as a main deposited N sink for the ecosystem. However, biogeochemical reactions responsible for N retention in the soil are still not fully understood. Moreover, the majority of the concerned studies were conducted in acidic or hydromorphous soils and very little is known today about the fate of N deposition under other soil physico-chemical conditions. Consequently, the present research aims at filling some of the gaps described above and deals with two main topics concerning the retention of atmospherically deposited N in soils that are (1) the characterisation of the retention of atmospherically deposited N in the soil, in terms of both duration and quantity, more specifically, in a well drained calcareous soil and (2) the mechanisms and processes responsible for such a retention. From a practical point of view, the retention of N deposition in the soil was characterised by means of a 15N field labelling experiment simulating N atmospheric deposition and conducted at the Grandvillard research site, in the riparian zone of La Sarine River (Swiss Prealps). The stand is a beech forest mixed with planted spruces. The soil consists of a well drained calcareous fluvisol with a fast organic matter turnover. We tracked the N tracers (15NH4+ or 15NO3-, corresponding to the main forms of deposition) from short-term (hours, days or weeks) to longer-term intervals (one year), by measuring the partitioning of 15N into different biochemical soil fractions (extractable N, microbial N, roots N and N immobilised in soil). Aiming at studying the mechanisms and processes responsible for N retention, we performed two laboratory experiments. The first one was an acid hydrolysis, in order to determine the