Weathering and atmospheric deposition signatures of base cations in upland soils of NE Scotland: their application to critical load assessment

Although the importance of atmospheric base cation deposition to soils is increasingly acknowledged, field data demonstrating the signatures of atmospheric and weathering-derived cations are seldom presented. This paper explores how combined atmospheric and internal base cation inputs are reflected in exchangeable and soil solution base cation compositions of upland soils, ranging from acidic to basic parent materials. Although total atmospheric base cation deposition exceeded weathering inputs for most soils, relative proportions of exchangeable base cations were controlled by parent materials. Calcium and magnesium concentrations were most closely related to geology, but were likely affected by the affinity of humic material for divalent cations, which resulted in elevated concentrations in surface horizons. In contrast, soil solution compositions deviated little from those of precipitation, showing minimal geochemical influence. A soil-based weathering rate index utilizing the dominance of the base cation Na was explored with the goal of enhancing spatial resolution in critical load assessment. However, the calibration of the relative proportions of exchange site Na against weathering rates, determined by three conventional critical load calculation methods, was inconclusive. Although power relationships were suggested, these were poor and largely insignificant, with a considerable range of %Na values at low weathering rates. It was not resolved whether the data available was insufficient to obtain accurate weathering rates, or that the base cation chemistry was affected by factors other than weathering alone.