The importance of mineral determinations to PROFILE base cation weathering release rates: a case study

Abstract

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.

Weathering rates in Swedish forest soils

“…the mass balances. The high weathering rates produced by the mass balances, especially in Asa, were largely explained by the measured depletion of base cations in the soil being much lower than the accumulation in the young Norway spruce stands (Casetou-Gustafson et al, 2019b). The very high estimated weathering rates, especially in Asa, indicate that flows are described inadequately.…”

“…The very high estimated weathering rates, especially in Asa, indicate that flows are described inadequately. Uptake occurring below the defined rooting zone could be one contributing factor (Casetou-Gustafson et al, 2019b). In Asa, very low weathering rates from the depletion method contributed to the wide span.…”

“…In Asa, very low weathering rates from the depletion method contributed to the wide span. The low weathering rates originated from a fairly flat Zr depth gradient in the soil, which indicates that the soil had probably been disturbed, so the necessary assumptions for the depletion method were not satisfied (Casetou-Gustafson et al, 2019b).…”

“…The programme, which focused on weathering rates for the base cations Ca, Mg, Na and K in Sweden, included approaches covering the whole spectra from laboratory-scale experiments, through plot-and catchment-scale experiments in the field, to extensive weathering modelling. Different approaches were compared at different scales and were in some cases refined (Stendahl et al, 2013;Casetou-Gustafson et al, 2018, 2019aBelyazid et al, 2019;Kronnäs et al, 2019;Erlandsson et al, 2016). The input data were also examined for uncertainties relating to the generalisations made when estimating normative mineralogy based on total chemical analyses (Casetou-Gustafson et al, 2018, 2019a.…”

“…Different approaches were compared at different scales and were in some cases refined (Stendahl et al, 2013;Casetou-Gustafson et al, 2018, 2019aBelyazid et al, 2019;Kronnäs et al, 2019;Erlandsson et al, 2016). The input data were also examined for uncertainties relating to the generalisations made when estimating normative mineralogy based on total chemical analyses (Casetou-Gustafson et al, 2018, 2019a. Two other potential sources of uncertainties that have been explored, but that are still widely discussed, were revisited: (1) the role of biological weathering that might generate weathering not included in the current generation of biogeochemical models (Banfield et al, 1999;Finlay et al, 2009Finlay et al, , 2019 and (2) model simplifications related to base cation exchange and aluminium complexation (Gustafsson et al, 2018;van der Heijden et al, 2018).…”