The Fate of [15N]Ammonium and [15N]Nitrate in the Soil of a 140-Year-Old Spruce Stand (Picea Abies) in the Fichtelgebirge (NE-Bavaria)

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Abstract
A 15N tracer-experiment was carried out in a 140-year-old spruce stand (Picea abies (L.) Karst.) in the Fichtelgebirge (NE-Bavaria, Germany). Highly enriched (98 at%) [15N]ammonium and [15N]nitrate were applied as tracers by simulation of a deposition of 41.3 mol N ha−1 with 11 water m−2. To examine seasonal variations of uptake by spruce and understorey vegetation, different plots were labelled in spring, summer and autumn 1994. One aim of the present study was to perfect a method of preparation of soil extracts for isotope ratio mass spectrometry (IRMS) measurements. Ammonium and nitrate from soil extracts were prepared for IRMS measurements by steam distillation and subsequent freeze drying. Additionally, tracer distribution and transformations in the soil nitrogen pools were examined. Ammonium, nitrate and total nitrogen were examined in the organic layer and the upper 10 cm of the mineral soil during 3 months after the first tracer application in spring 1994. In July 1994, three months after tracer application, 40% of the [15N]ammonium label and 29% of the [15N]nitrate label, respectively, were recovered in the total N pool of the investigated soil horizons. In the organic layer the L/Of horizon retained most of the recovered tracers. Nitrification, immobilisation and mineralisation occurred even under the conditions of high soil acidity at the study site.