The importance of mineral determinations to PROFILE base cation weathering rates

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 program based on geochemical data.
The aim of this study was to investigate how uncertainties in quantitative
mineralogy, originating from modelled mineral abundance and assumed
stoichiometry, influence PROFILE weathering estimates, 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 “Analysis
to Mineralogy” program ('A2M') using two sets of qualitative mineralogical data inputs to
A2M: A site-specific mineralogy
calculated based on information about mineral identity and chemical composition
as determined by XRPD and electron microprobe analysis (EMPA) and regional mineralogy, representing the assumed mineral
identity and compositions for larger geographical areas in Sweden. For the site-specific mineral
input the precise
elemental compositions of minerals were determined by microprobe
analysis, whereas for the regional
mineralogy the compositions were as assumed in previous
studies. Arithmetic means of WA2M were generally in relatively
close agreement with WXRPD. The data
available here contains the PROFILE model outcome for the different mineralogies,
and the root-mean square errors of A2M-based PROFILE weathering rates compared
to the XRPD based weathering rates.