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-2yr-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-2yr-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-2yr-1,
to mean levels of 7-70 meq m-2yr-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.
Keywords: Lakes, hydrochemistry, nitrogen, nitrate, sinks, leaching, acidification,
critical loads, FAB model |