Present and potential nitrogen outputs from Norwegian soft water lakes – an assessment made by applying the steady-state First- order Acidity Balance (FAB) model

The steady-state First-order Acidity Balance (FAB) model for calculating critical loads of sulphur (S) and nitrogen (N) is applied to 609 Norwegian soft-water lakes to assess the future nitrate (NO3 -) leaching potential under present (1992-96) S and N deposition. The lakes were separated into five groups receiving increasing levels of N deposition (<25, 25-49, 50-74, 75-99 and 100-125 meq m-2 yr-1). Using long-term sustainable N sink rates presently recommended for FAB model applications, N immobilisation, net N uptake in forests, denitrification and in-lake N retention were estimated for each group of lakes. Altogether, the long-term N sinks constituted 9.9±3.2 to 40.5±11.4 meq m-2 yr-1 in the lowest and highest N deposition categories, respectively. At most sites, the current N deposition exceeds the amount of N retained by long-term sustainable N sinks plus the NO3 loss via the lake outlets. This excess N, which is currently retained within the catchments may, according to the FAB model, leach as acidifying NO3 in the future. If these predictions are fulfilled, NO3 leaching at sites in the various N deposition categories will increase dramatically from present (1995) mean levels of 1-20 meq m-2 yr-1, to mean levels of 7-70 meq m-2 yr-1 at future steady state. To illustrate the significance of such an increase in NO3 leaching, the mean Acid Neutralising Capacity (ANC) at sites in the highest N deposition category may decrease from -18±15 μeq L-1 at present, to -40±20 μeq L-1. Under present S and N deposition levels, the FAB model predicts that 46% of the Norwegian lakes may experience exceedances of critical loads for acidifying deposition. In comparison, the Steady-State Water Chemistry model (SSWC), which considers only the present N leaching level, estimates critical load exceedances in 37% of the lakes under the same deposition level. Thus far, there are great uncertainties regarding both the time scales and the extent of future N leaching, and it is largely unknown whether the FAB model predictions will ever be fulfilled. Hence, long-term monitoring and further studies on N immobilisation processes under varying N deposition levels and ecosystem types seem necessary to make better predictions of future NO3 leaching.