Contribution of nitrogen to acidity in the Bjerkreim River in southwestern Norway

In the acidified areas of southern Norway, sulfate deposition has decreased by about 40% since 1980, while nitrogen deposition has been constant during the same period. The wet deposition of nitrogen today is about twice that of sulfur. Consequently, the relative importance of nitrogen for the acidification of surface waters and soils has increased. In the Bjerkreim river catchment (southwestern Norway), nitrogen is an important acidifying component. In the upper part of the catchment, deposition of S and N must be reduced substantially in order to reach nonexceedance of critical loads in the runoff waters. After the 1994 Sulphur Protocol is implemented, nitrogen will represent the major part of the exceedance, and substantial reductions of N depositions will be necessary. In the most sensitive subcatchment both N and S deposition must be reduced by more than 80% of present deposition in order to achieve nonexceedance in runoff. Since this catchment is representative of large parts of the acidified areas in Norway, further decreases in acidification of surface waters could be more effectively accomplished by reductions of N rather than of S. The concentrations of P in lakes and rivers in the Bjerkreim catchment are extremely low and are frequently below analytical detection limits$(1\ \mu {\rm g}\ {\rm L}^{-1})$. This is partly due to the geology and dominance (56%) of mountainous and highland areas. The thin soils and steep topography in these areas result in rapid water flow in response to precipitation. The catchment characteristics have consequences not only for acidification sensitivity, but also for the ability to retain N from atmospheric inputs. The combined effect of high N deposition, high amounts of precipitation and the low P content of soils and surface waters suggests that the high NO3concentrations in runoff could reflect P limitation in the soils in the nonforested parts of the Bjerkreim catchment.