Long-term behaviour of 15 N in an alpine grassland ecosystem MARTIN H. GERZABEK 1,2, *, GEORG HABERHAUER 1 , MICHAEL STEMMER 2 , SABINE KLEPSCH 2 and ERNST HAUNOLD 3 1 Department of Environmental Research, ARC Seibersdorf research, A-2444 Seibersdorf, Austria 2 Institute of Soil Research, University of Natural Resources and Applied Life Sciences, Vienna, Gregor- Mendel-Strasse 33, A-1180 Vienna, Austria; 3 Am Platengrund 5e, A-2345 Brunn am Gebirge, Austria; *Author for correspondence (e-mail: martin.gerzabek@boku.ac.at; phone: þ43-1-47654-3117; fax: þ43-1-4789110) Received 9 May 2003; accepted in revised form 9 October 2003 Key words: Alpine grassland, Cambisol, 15 N, N dynamics, N-pools Abstract. Nitrogen dynamics in semi-natural environments is crucial for the development and ecological stability of these systems. The present paper shows the results of the reinvestigation of a 15 N-tracer experiment, which was established in the Grossglockner massif in Austria at 2300 m a.s.l. in 1974=1975. We show that large quantities of nitrogen introduced by a single pulse labelling (amounting to approxi- mately 1.7% of the nitrogen in the system) into an alpine grassland remain in the soil–plant system, with only 55% being lost during 27–28 years. In the first 10 cm of the four investigated soil profiles 40% of 15 N was recovered, being mainly bound in organic forms. A simple site specific model was established on the basis of the results considering a biological, residual and labile N-pool, the latter being the source for N-losses. By the model a long mean residence time close to 100 years was derived for the remaining 15 N. Introduction The global nitrogen (N) cycle is increasingly influenced by industrial emissions (Nadelhoffer et al. 1999) resulting in N depositions on soils in the Alps of 10 kg ha 1 year 1 or more, exceeding natural net N-mineralization (Ko ¨rner 1999). In contrast to intensive agricultural ecosystems, characterised by large N-input and N-losses, semi-natural environments like forests or alpine=arctic grasslands, often being deficient in plant nutrients in general, are characterised by an effective bio- logical recycling mechanism (Hackl et al. 2000). Productivity of alpine grasslands may be limited by the size of the available N pool, N-deposition and N-fixation by free living organisms being the only input (Ko ¨rner 1999). In these ecosystems N-input through deposition might have a sig- nificant impact by altering the rate of the internal nitrogen cycle governed by turnover times of live and dead aboveground and belowground plant tissues and soil micro-organisms (Ko ¨ rner 1999; Lipson et al. 1999). Short and medium-term studies with 15 N-labelled compounds elucidated already the dynamics of competition be- tween plants and soil micro-organisms (Jaeger et al. 1999). However, little is known about the mean residence times of N in the root zone or soil profiles in alpine ecosystems in general. Thus, predictions of the long-term effects of anthropogenic # 2004 Kluwer Academic Publishers. Printed in the Netherlands. Biogeochemistry 70: 59–69, 2004.