Devenir de l'azote minéral dans une tourbière à Sphagnum fallax Klinggr. et Carex rostrata Stokes du Massif central (France)

The fate of mineral nitrogen (N) in an oligotrophic fen, located in Monts de Forez (Puy-de-Dôme department), was studied by 15N tracking of 0.5 g (nitrogen)·m-2·year-1 ammonitrate labelled on both ions (30% excess). Simulating atmospheric deposits, an important nutrient source in mires, N additions were made at two periods of the year, June and August, 1986, and the fate of the nitrogen was measured in October of the same year. Nitrogen recovery was measured in the living parts of Sphagnum fallax Klinggr., aerial and hypogeous parts of Carex rostrata Stokes, and in peat. Vegetation preferably absorbed nitrogen in June, as compared to August, the period during which nitrogen allocation to hypogeous phytomasses of Carex was the highest. As compared to Carex, Sphagnum has strong nitrogen assimilation and holding potentials. A nitrogen competition potential index or "nitrogen sink power" (PCN), calculated from recovery and stock data, showed that Sphagnum was more efficient at intercepting nitrogen in August, when rainfall is the lowest in the studied area. In peat, nitrogen recovery was strongest in August. Most of the microbial processes seem to occur in the first 10 cm. Gross nitrogen mineralization and immobilization, calculated from organic matter degradation, were estimated as 11.4 and 6.9 g·m-2·year-1, respectively. The gross mineralization estimation according to the method of Fried and Dean was 17.2 g·m-2·year-1. This overestimation might be attributed to an underestimation of the nitrogen derived from input in plants and an overestimation of nitrogen from peat.Key words: 15N, nitrogen balance, nitrogen uptake, competition, retention, turfigenous process.