In the Speuld forest, the Netherlands, the dynamic soil acidification model NUCSAM has been applied to a manipulation experiment in which part of the forest was roofed to control nitrogen (N) and sulphur (S) deposition. The roofed area was divided into two subplots: one received ambient N and S deposition and one with pristine N and S deposition. Concentration measurements on each plot showed a high (time-dependent) spatial variability. Statistical analyses of the concentrations on both subplots showed small but significant, effects of the deposition reduction on nitrate (NO3), sulphate (SO4 ) and aluminium (Al) concentrations. The statistical significance of the effects was minimised by the large spatial variability within the plots. Despite these shortcomings it can be concluded that simulated concentrations were generally within the 95 % confidence interval of the measurements although the effect of a reduction in N deposition on soil solution chemistry was underestimated due to a too strong decline in N-uptake.
An overview of the measured and simulated concentrations of the main solutes at 10, 90 cm depth is given in Figure 2a and Figure 2b. The results were obtained almost without calibration of the process parameters describing the bio-geochemical interactions. The slight underestimation of the measured Cl concentrations by NUCSAM at 10 cm depth, was also found for the other concentrations at this depth. The average deviation from the confidence interval was however relatively small, except for NO3, Al and NH4. pH values were exceptionally well simulated by the model, most values were within the 95% confidence interval and the C-index was 1.002 and the RMSE was smaller or equal to the standard error of the observations. The results at 90 cm depth corresponded generally better with the measurements than in the topsoil, which coincides with the better hydrological simulation results (see Cl concentrations). Just as in the topsoil NO3 and Al concentrations were overestimated during the summer period, especially at 90 cm depth during 1990 and 1991. NUCSAM also overestimated divalent base cations during this period, probably due to the exchange of Ca against Al on the adsorption complex.
The performance of NUCSAM to simulate the measured concentrations on the plot with clean deposition was mostly similar to the results for the plot with ambient deposition. Comparison of the observations on the roofed plots with ambient and pristine deposition showed that deposition reduction led to a slightly significant decrease in SO4, NO3 and Al. Application of NUCSAM also showed that deposition reduction clearly led to lower simulated NO3, SO4 and Al concentrations at all depth (Figure 4a and Figure 4b). NUCSAM overestimated the reduction in SO4 concentrations at 90 cm depth. At 10 and 45 cm depth measured concentrations changes were rather well simulated. The average reduction in NO3 (and Al) concentrations is overestimated at 10 and 90 cm depth. The overestimation is mainly due to the strong reduction in the height of the summer NO3 peaks, whereas the effect on the low winter concentrations appears to be underestimated by NUCSAM. Conclusions based on these results should be cautiously drawn because inherent differences do exist between the two subplots due to spatial variability. These differences could not be taken into account because they are difficult to quantify due to the lack of an initial period in which both subplots are treated in the same way.
