A two-layer application of the MAGIC model to predict the effects of land use scenarios and reductions in deposition on acid sensitive soils in the UK

A two-layer application of the catchment-based soil and surface water acidi fication model, MAGIC, was applied to 21 sites in the UK Acid Waters Monito ring Network (AWMN), and the results were compared with those from a one-la yer application of the model. The two-layer model represented typical soil properties more accurately by segregating the organic and mineral horizons into two separate soil compartments. Reductions in sulphur (S) emissions as sociated with the Second S Protocol and different forestry (land use) scena rios were modelled, and their effects on soil acidification evaluated. Soil acidification was assessed in terms of base saturation and critical loads for the molar ratio of base cations (Ca2+ + Mg2+ + K+) to aluminium (Al) in soil solution. The results of the two-layer application indicate that base saturation of the organic compartment was very responsive to changes in la nd use and deposition compared with the mineral soil. With the two-layer mo del, the organic soil compartment was particularly sensitive to acid deposi tion, which resulted in the critical load being predicted to be exceeded at eight sites in 1997 and two sites in 2010. These results indicate that fur ther reductions in S deposition are necessary to raise the base cation (BC) :Al ratio above the threshold which is harmful to tree roots. At forested s ites BC:AI ratios were generally well below the threshold designated for so il critical loads in Europe and forecasts indicate that forest replanting c an adversely affect the acid status of sensitive organic soils. This illust rates the importance of proactive and responsible forestry management polic ies consistent with the longer term objectives of protecting and sustaining soil and water quality. Policy formulation must seek to protect the most s ensitive environmental receptor, in this case organic soils. It is clear, t herefore, that simply securing protection of surface waters, via the critic al loads approach, may not ensure adequate protection of low base status or ganic soils from the effects of acidification.