Accurate estimates of base cation weathering rates in forest soils are crucial for policy decisions on sustainable biomass harvest levels and for calculations of critical loads of acidity. The PROFILE model is one of the most frequently used methods to quantify weathering rates, where the quantitative mineralogical input has often been calculated by the A2M ("Analysis to Mineralogy") program based solely on geochemical data. The aim of this study was to investigate how uncertainties in quantitative mineralogy, originating from modeled mineral abundance and assumed stoichiometry, influence PROFILE weathering estimate, by using measured quantitative mineralogy by X-ray powder diffraction (XRPD) as a reference. Weathering rates were determined for two sites, one in northern (Flakaliden) and one in southern (Asa) Sweden. At each site, 3-4 soil profiles were analyzed at 10 cm depth intervals. Normative quantitative mineralogy was calculated from geochemical data and qualitative mineral data with the A2M program using two sets of qualitative mineralogical data inputs to A2M: (1) a site-specific mineralogy based on information about mineral identification and mineral chemical composition as determined directly by XRPD and electron microprobe analysis (EMPA), and (2) regional mineralogy, representing the assumed minerals present and assumed mineral chemical compositions for large geographical areas in Sweden, as per previous published studies. Arithmetic means of the weathering rates determined from A2M inputs (W A2M ) were generally in relatively close agreement with those (W XRPD ) determined by inputs based on direct XRPD and EMPA measurements. The hypothesis that using site-specific instead of regional mineralogy will improve the confidence in mineral data input to PROFILE was supported for Flakaliden. However, at Asa, site-specific mineralogies reduced the discrepancy for Na between W A2M and W XRPD but produced larger and significant discrepancies for K, Ca and Mg. For Ca and Mg the differences between weathering rates based on different mineralogies could be explained by differences in the content of some specific Ca-and Mg-bearing minerals, in particular amphibole, apatite, pyroxene and illite. Improving the accuracy in the determination of these minerals would reduce weathering uncertainties. High uncertainties in mineralogy, due for example to different A2M assumptions, had surprisingly little effect on the predicted weathering of Na-and K-bearing minerals. This can be explained by the fact that the weathering rate constants for the minerals involved, e.g. K feldspar and micas, are similar in PROFILE. Improving the description of the dissolution rate kinetics of the plagioclase mineral group as well as major K-bearing minerals (K feldspars and micas) should be a priority to help improve future weathering estimates with the PROFILE model.
<p><strong>Abstract.</strong> 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.</p> <p>In the 0&#8211;50&#8201;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&#8201;mmol<sub>c</sub>&#8201;m<sup>&#8722;2</sup>&#8201;yr<sup>&#8722;1</sup>, respectively. Corresponding values at Flakaliden were 7.3, 9.0, 1.7 and 4.4&#8201;mmol<sub>c</sub>&#8201;m<sup>&#8722;2</sup>&#8201;yr<sup>&#8722;1</sup>, respectively. Steady state weathering rate for Ca, Mg, K and Na estimated with PROFILE was 8.9, 3.8, 5.9 and 18.5&#8201;mmol<sub>c</sub>&#8201;m<sup>&#8722;2</sup>&#8201;yr<sup>&#8722;1</sup>, respectively, at Asa and 11.9, 6.7, 6.6 and 17.5&#8201;mmol<sub>c</sub>&#8201;m<sup>&#8722;2</sup>&#8201;yr<sup>&#8722;1</sup>, 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&#8201;cm (Asa) and 60&#8201;cm (Flakaliden). In contrast, the depletion method indicated that the largest postglacial losses were in upper soil layers, particularly at Flakaliden.</p> <p>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&#8201;mmol<sub>c</sub>&#8201;m<sup>&#8722;2</sup>&#8201;yr<sup>&#8722;1</sup> at Asa and 35, 14 and 22&#8201;mmol<sub>c</sub>&#8201;m<sup>&#8722;2</sup>&#8201;yr<sup>&#8722;1</sup> at Flakaliden), but lower Na weathering rates similar to the depletion method (6.6 and 2.2&#8201;mmol<sub>c</sub>&#8201;m<sup>&#8722;2</sup>&#8201;yr<sup>&#8722;1</sup> 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.</p>
We will try to simplify the abstract. Introduction: We agree with the referee that it was an oversight not to refer to the work by Hodson et al. (1997) and we will cite it in the introduction. We also thank the referee for pointing out the difficulty of understanding hypothesis 1. We would like to clarify that site-specific mineralogy is determined in terms of the minerals identified and the chemical compositions of these minerals but not determined (directly) in terms of the abundance of these minerals, which is instead calculated by A2M. Although the definitions of 'site specific', 'regional' and 'measured' mineralogy are C1
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