From minerals to trees: Reducing uncertainty in estimating weathering rates for sustainable forest growth

Sustainable forest growth is fundamentally dependent on long-term supply of nutrients from mineral dissolution. Over the past century, losses of base cations (Ca, K, Mg, Na) from forest soils have been accelerated through increased forest production and anthropogenic acidification. Substantial uncertainties about the accuracy in weathering rate estimates has hampered designation of appropriate levels for maintaining sustainable forest production. The aim of this thesis was to improve the accuracy of estimates by using better mineralogical data inputs to a frequently applied weathering estimation model (PROFILE) and by comparing weathering rates derived from three different methods that differ conceptually, based on harmonised input data and boundary conditions. A new site-specific approach for calculating indirect quantitative soil mineralogy with the “Analysis to Mineralogy” (‘A2M’) model was developed, by direct determination of the mineralogical input data to A2M. Overall, A2M-derived quantitative mineralogy reproduced the main pattern of “reference” mineral contents (measured directly by X-ray powder diffraction (XRPD)). An exception to this was observed for major K-bearing minerals in the regional quantitative A2M mineralogy. Regardless of these observed discrepancies, PROFILE weathering estimates were uncertain irrespective of the type of mineralogical input data that was used. For Ca and Mg, uncertainties arose from lack of knowledge about presence or absence of specific minerals characterised by high dissolution rates (i.e. apatite, amphibole, pyroxene, calcite and illite). For K and Na, uncertainties related to use of inaccurate dissolution rate coefficients for dioctahedral mica and plagioclase. Overall, the PROFILE model and the depletion method were shown to estimate present-day and long-term weathering rates of Ca and Mg with reasonable accuracy. Comparisons of weathering rates in a base cation budget showed that trees take up base cations from forest soils, at least when growing intensively, probably from sources other than weathering