The importance of marine-derived base cations and sulphur in estimating critical loads in Scotland

Soil samples have been collected from major horizons of Scottish upland Calluna moorland podzols derived from granites. The soil was subjected to routine chemical and mineralogical analysis. The PROFILE model was then used to calculate soil weathering rates, and hence critical loads and exceedance for sulphur depositon. Soil pH was found to be significantly correlated with marine base cation deposition. It is shown that this probably reflects the greater size of marine-derived base cation deposition fluxes compared with weathering-derived base cation fluxes estimated using PROFILE. Only when total wet deposited base cation inputs are used in addition to weathering rates as a basis for assessing critical loads and exceedance values are strong correlations found between soil pH values and exceedance for these sensitive soils. If modelled divalent base cations to aluminium ratios in soil solution are to be used as a basis for quantifying soil damage, all base cation input effects need to be considered, and not just non-marine inputs.