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. Reliable and accurate methods for estimating soil mineral weathering ratesare required tools in evaluating the sustainability of increased harvestingof forest biomass and assessments of critical loads of acidity. A variety ofmethods that differ in concept, temporal and spatial scale, and datarequirements are available for measuring weathering rates. In this study,causes of discrepancies in weathering rates between methods were analysedand were classified as being either conceptual (inevitable) or random. Therelease rates of base cations (BCs; Ca, Mg, K, Na) by weathering wereestimated in podzolised glacial tills at two experimental forest sites, Asaand Flakaliden, in southern and northern Sweden, respectively. Threedifferent methods were used: (i) historical weathering since deglaciationestimated by the depletion method, using Zr as the assumed inert reference; (ii) steady-state weathering rate estimated with the PROFILE model, based onquantitative analysis of soil mineralogy; and (iii) BC budget at standscale, using measured deposition, leaching and changes in base cation stocksin biomass and soil over a period of 12 years. In the 0–50 cm soil horizonhistorical weathering of BCs was 10.6 and 34.1 mmol c  m −2  yr −1 , at Asa and Flakaliden, respectively. Corresponding values ofPROFILE weathering rates were 37.1 and 42.7 mmol c  m −2  yr −1 .The PROFILE results indicated that steady-state weathering rateincreased with soil depth as a function of exposed mineral surface area,reaching a maximum rate at 80 cm (Asa) and 60 cm (Flakaliden). In contrast,the depletion method indicated that the largest postglacial losses were inupper soil horizons, particularly at Flakaliden. With the exception of Mg and Ca in shallow soil horizons, PROFILE producedhigher weathering rates than the depletion method, particularly of K and Nain deeper soil horizons. The lower weathering rates of the depletion methodwere partly explained by natural and anthropogenic variability in Zrgradients. The base cation budget approach produced significantly higherweathering rates of BCs, 134.6 mmol c  m −2  yr −1 at Asa and73.2 mmol c  m −2  yr −1 at Flakaliden, due to high ratesestimated for the nutrient elements Ca, Mg and K, whereas weathering rateswere lower and similar to those for the depletion method (6.6 and 2.2 mmol c  m −2  yr −1 at Asa and Flakaliden). The large discrepancy inweathering rates for Ca, Mg and K between the base cation budget approachand the other methods suggests additional sources for tree uptake in the soilnot captured by measurements.

biogeosciences

Current, steady-state and historical weathering rates of base cations at two forest sites in northern and southern Sweden: a comparison of three methods