Stable isotope analysis of 15N/14N and 18O/16O-nitrate was used to investigate the nitrate dynamics and potential groundwater pollution in an Alpine forest stand in Tyrol/Austria. The dynamics of Delta15-Nnitrate values were followed in a forest ecosystem. The stable isotopic values of the throughfall are comparable with other studies. The completely decoupled dynamics of the Delta15-Nnitrate of the precipitation and the surface water was observed. High variations in Delta15N - nitrate values in rainfall indicate that nitrate of different sources is deposted at that site. A significant correlation between the Delta 15 Nnitrate values of the surface water and soil water was obtained, while no significant correlation between the Delta 15Nnitrate values of any precipitation sample with the suface water could be found. This suggests that the main source of nitrate in soil water originates from microbiological activity such as nitrification reactions and less from nitrate input by deposition. The results of Delta18Onitrate measurements strongly supported the microbiological origin of nirate in the surface and soil water. In an additional lysimeter experiment, 15N - labelled nitrate was applied to study nitrate transport in soil. After 130 days and the collection of 300 L leachate, a total of 52% of the applied nitrate was detected in seepage water.
