The fate of mineral nitorgen in a fen with sphagnum fallax klinggr. and Carex rostrata stokes (Massif-central, France)

The fate of mineral nitrogen (N) in an oligotrophic fen, located in Monts d e Forez (Puy-de-D.me department), was studied by N-15 tracking of 0.5 g (ni trogen) m(-2) year(-1) ammonitrate labelled on both ions (30% excess). Simu lating atmospheric deposits, an important nutrient source in mires, N addit ions were made at two periods of the year, June and August, 1986, and the f ate of the nitrogen was measured in October of the same year. Nitrogen reco very was measured in the living parts of Sphagnum fallax Klinggr., aerial a nd hypogeous parts of Carex rostrata Stokes, and in peat. Vegetation prefer ably absorbed nitrogen in June, as compared to August, the period during wh ich 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 low est 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 n itrogen mineralization and immobilization, calculated from organic matter d egradation, 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 De an 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 overes timation of nitrogen from peat.