Evaluating critical loads of acidity for Swiss forest soils: Comparison of two calculation methods

Abstract

A variant of the European model Simple Mass Balance (SMB) and the regionalized PROFILE model have been used to calculate critical loads of acidity for Swiss forest soils. The single layer SMB has been applied to 11,800 receptor points and the multi-layer PROFILE to 720 forest sites. Weathering rates used in SMB calculations were assessed by means of a modified de Vries soil classification, and calculated from physical properties of the soil system with PROFILE. Cumulative frequency distributions of the results at the national resolution show that PROFILE predicts lower critical loads. Up to the 65-percentile critical load PROFILE percentile values are average 65% of the SMB percentile values. The upper percentiles of the PROFILE critical loads are merely 25% of the respective SMB predictions. The analysis of the model predictions on a regional scale implies that the multi-layer model should be used for the assessment of critical loads for areas with potentially calcareous forest soils. The inherent inability of the SMB method to properly account for processes in the carbonate system and to estimate adequate weathering rates results in significantly higher critical loads compared to PROFILE predictions. Both models estimate critical loads in better agreement for low weathering forest soils in high-precipitation areas where hydrogen and aluminum leaching govern the model result.