CONTRIBUTION OF NITROGEN TO ACIDITY IN THE BJERKREIM RIVER IN SOUTHWESTERN NORWAY

In the acidified areas of southern Norway, sulfate deposition has decr eased by about 40% since 1980, while nitrogen deposition has been cons tant during the same period. The wet deposition of nitrogen today is a bout twice that of sulfur. Consequently, the relative importance of ni trogen 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 par t of the exceedance, and substantial reductions of N depositions will be necessary. In the most sensitive subcatchment both N and S depositi on must be reduced by more than 80% of present deposition in order to achieve nonexceedance in runoff. Since this catchment is representativ e of large parts of the acidified areas in Norway, further decreases i n acidification of surface waters could be more effectively accomplish ed by reductions of N rather than of S. The concentrations of P in lak es and rivers in the Bjerkreim catchment are extremely low and are fre quently below analytical detection limits (1 mu g L-1). This is partly due to the geology and dominance (56%) of mountainous and highland ar eas. The thin soils and steep topography in these areas result in rapi d water flow in response to precipitation. The catchment characteristi cs have consequences not only for acidification sensitivity, but also for the ability to retain N from atmospheric inputs. The combined effe ct of high N deposition, high amounts of precipitation and the low P c ontent of soils and surface waters suggests that the high NO3 concentr ations in runoff could reflect P limitation in the soils in the nonfor ested parts of the Bjerkreim catchment.