Winter climate affects long-term trends in stream water nitrate in acid-sensitive catchments in southern Norway

摘要

Controls of stream water NO₃ in mountainous and forested catchments arenot thoroughly understood. Long-term trends in stream water NO₃ arepositive, neutral and negative, often apparently independent of trends in Ndeposition. Here, time series of NO₃ in four small acid-sensitivecatchments in southern Norway were analysed in order to identify likelydrivers of long-term changes in NO₃. In two sites, stream waterNO₃ export declined ca 50% over a period of 25 years while in theother sites NO₃ export increased with roughly 20%. Discharge and Ndeposition alone were poor predictors of these trends. The most distincttrends in NO₃ were found in winter and spring. Empirical modelsexplained between 45% and 61% of the variation in weeklyconcentrations of NO₃, and described both upward and downward seasonaltrends tolerably well. Key explaining variables were snow depth, discharge,temperature and N deposition. All catchments showed reductions in snow depthand increases in winter discharge. In two inland catchments, located inmoderate N deposition areas, these climatic changes appeared to drive thedistinct decreases in winter and spring concentrations and fluxes ofNO₃. In a coast-near mountainous catchment in a low N deposition area,these climatic changes appeared to have the opposite effect, i.e. lead toincreases in especially winter NO₃. This suggests that the effect of areduced snow pack may result in both decreased and increased catchment Nleaching depending on interactions with N deposition, soil temperatureregime and winter discharge.