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

Reliable and accurate methods for estimating soil mineral weathering rates are required tools in evaluating the sustainability of increased harvesting of forest biomass and assessments of critical loads of acidity. A variety of methods that differ in concept, temporal and spatial scale, and data requirements are available for measuring weathering rates. In this study, causes of discrepancies in weathering rates between methods were analysed and were classified as being either conceptual (inevitable) or random. The release rates of base cations (BCs; Ca, Mg, K, Na) by weathering were estimated in podzolised glacial tills at two experimental forest sites, Asa and Flakaliden, in southern and northern Sweden, respectively. Three different methods were used: (i) historical weathering since deglaciation estimated by the depletion method, using Zr as the assumed inert reference; (ii) steady-state weathering rate estimated with the PROFILE model, based on quantitative analysis of soil mineralogy; and (iii) BC budget at stand scale, using measured deposition, leaching and changes in base cation stocks in biomass and soil over a period of 12 years. In the 050 cm soil horizon historical weathering of BCs was 10.6 and 34.1 mmolc m(-2) yr(-1), at Asa and Flakaliden, respectively. Corresponding values of PROFILE weathering rates were 37.1 and 42.7 mmolc m(-2) yr(-1). The PROFILE results indi- cated that steady-state weathering rate increased 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 in upper soil horizons, particularly at Flakaliden.With the exception of Mg and Ca in shallow soil horizons, PROFILE produced higher weathering rates than the depletion method, particularly of K and Na in deeper soil horizons. The lower weathering rates of the depletion method were partly explained by natural and anthropogenic variability in Zr gradients. The base cation budget approach produced significantly higher weathering rates of BCs, 134.6 mmolc m(-2) yr(-1) at Asa and 73.2 mmolc m(-2) yr(-1) at Flakaliden, due to high rates estimated for the nutrient elements Ca, Mg and K, whereas weathering rates were lower and similar to those for the depletion method (6.6 and 2.2 mmolc m(-2) yr(-1) at Asa and Flakaliden). The large discrepancy in weathering rates for Ca, Mg and K between the base cation budget approach and the other methods suggests additional sources for tree uptake in the soil not captured by measurements