Comparison of simulated forest soil response to acid deposition reduction with two models of differing complexity

Great effort has been dedicated to developing soil acidification models for use on different scales. This paper focuses on the changes in model perfor mance of a site scale soil acidification model (NUCSAM) and a national to E uropean scale soil acidification model (SMART 2). This was done to gain ins ight into the effects of model simplification. Because these models aim to predict the response to reduction in acid deposition, these models must be tested under such circumstances. A straightforward calibration and validati on of the regional model, however, is hampered by lack of observations over a sufficient time period. Consequently, NUCSAM was calibrated and validate d to a manipulation experiment involving reduced acid deposition in the Spe uld forest, the Netherlands. SMART 2 was then used with calibrated input da ta from NUCSAM. The acid deposition was excluded by a roof beneath the cano py. The roofed area consists of a plot receiving pristine deposition levels of nitrogen (N) and sulphur (S) and a control plot receiving ambient depos ition. NUCSAM was calibrated on the ambient plot, followed by a validation of both models on the pristine plot. Both models predicted soil solution concentrations within the 95% confidenc e interval of the observed responses for both the ambient plot and the pris tine plot at 90 cm depth. Despite the large seasonal and vertical (spatial) variation in soil solution chemistry, the trends in annual flux-weighted s oil solution chemistry, as predicted by SMART 2 and NUCSAM, corresponded we ll. The annual leaching fluxes below the root zone were also similar althou gh differences exist for the topsoil. For the topsoil, NUCSAM simulated the nutrients and acid related constituents better than SMART 2. Both models o verestimated the ammonium (NH4) concentration at 10 cm depth. SMART 2 under estimated calcium and magnesium (BC2+) concentration at 10 depth, whereas N UCSAM overestimated BC2+ concentration at 90 cm depth. NUCSAM predicted the effect of deposition reduction on N concentrations at both depths, whereas SMART 2 underestimated the effect of deposition reduction at 10 cm depth. Both models predicted faster effects of deposition reduction on aluminium ( Al), sulphate (SO4) and base cations than was observed. Generally, it appea red that the differences were large during the period of profound depositio n changes whereas small differences occurred during slight variations in de position level. It is concluded that a simpler model description does not a ffect the model's performance significantly as regards flux-weighted annual average concentrations at greater depth. Model improvements must focus on processes related to N-dynamics.