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

摘要

A two-layer application of the catchment-based soil andsurface water acidification model, MAGIC, was applied to 21 sites in the UK Acid WatersMonitoring Network (AWAMN), and the results were compared with those from a one-layerapplication of the model. The two-layer model represented typical soil properties moreaccurately by segregating the organic and mineral horizons into two separate soilcompartments. Reductions in sulphur (S) emissions associated with the Second S Protocoland different forestry (land use) scenarios were modelled, and their effects on soilacidification evaluated. Soil acidification was assessed in terms of base saturation andcritical loads for the molar ratio of base cations (CA2+ + MG 2+ + K+)to aluminium (Al) in soil solution. The results of the two-layer application indicate thatbase saturation of the organic compartment was very responsive to changes in land use anddeposition compared with the mineral soil. With the two- layer model, the organic soilcompartment was particularly sensitive to acid deposition, which resulted in the criticalload being predicted to be exceeded at eight sites in 1997 and two sites in 2010. Theseresults indicate that further reductions in S deposition are necessary to raise the basecation (BC):Al ratio above the threshold which is harmful to tree roots. At forested sitesBC:Al ratios were generally well below the threshold designated for soil critical loads inEurope and forecasts indicate that forest replanting can adversely affect the acid statusof sensitive term objectives of protecting and sustaining soil and water quality. Policyformulation must seek to protect the most sensitive environmental receptor, in this caseorganic soils. It is clear, therefore, that simply securing protection of surface waters,via the critical loads approach, may not ensure adequate protection of low base statusorganic soils from the effects of acidification.