Nitrate dynamics in an Alpine forest site (Mühleggerköpfl). O and N stable isotope analysis in natural water samples

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-N(nitrate) 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-N(nitrate) of the precipitation and the surface water was observed. High variations in delta15N-nitrate values in rainfall indicate that nitrate of different sources is deposited at that site. A significant correlation between the delta15N(nitrate) values of the surface water and soil water was obtained, while no significant correlation between the delta15N(nitrate) values of any precipitation sample with the surface 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 delta18O(nitrate) measurements strongly supported the microbiological origin of nitrate 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.