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

Abstract. Reliable and accurate methods for estimating soil mineral weathering rate are required tools in evaluating the sustainability of increased harvesting of forest biomass. A variety of methods that differ in concept, temporal and spatial scale and data requirements are available for measuring weathering rate. In this study, release rates of base cations through weathering were estimated in podsolised 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 with the depletion method, using Zr as assumed inert reference; (ii) steady-state weathering rate estimated with the PROFILE model, based on quantitative analysis of soil mineralogy; and (iii) base cation mass balance 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 0–50 cm soil layer at Asa, historical weathering of Ca, Mg, K and Na estimated by the depletion method was 4.7, 3.1, 0.8 and 2.0 mmol c  m −2  yr −1 , respectively. Corresponding values at Flakaliden were 7.3, 9.0, 1.7 and 4.4 mmol c  m −2  yr −1 , respectively. Steady state weathering rate for Ca, Mg, K and Na estimated with PROFILE was 8.9, 3.8, 5.9 and 18.5 mmol c  m −2  yr −1 , respectively, at Asa and 11.9, 6.7, 6.6 and 17.5 mmol c  m −2  yr −1 , respectively, at Flakaliden. Thus at both sites, the PROFILE results indicated 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 layers, particularly at Flakaliden. With the exception of Mg and Ca in shallow soil layers, PROFILE appeared to produce consistently higher weathering rates, particularly of K and Na in deeper soil layers. In contrast, the depletion method appeared to to produce consistently lower rather than higher weathering rates, due to natural and anthropogenic variability in (reference) Zr gradients. The mass balance approach produced significantly higher weathering rates of Ca, Mg, and K (65, 23, 40 mmol c  m −2  yr −1 at Asa and 35, 14 and 22 mmol c  m −2  yr −1 at Flakaliden), but lower Na weathering rates similar to the depletion method (6.6 and 2.2 mmol c  m −2  yr −1 at Asa and Flakaliden). The large discrepancy in weathering rates for Ca, Mg and K between mass balance and the other methods suggest that there were additional sources for tree uptake in the soil besides weathering and measured depletion in exchangeable base cations.