Abstract. Reliable and accurate methods for estimating soil mineral weathering rates
are required tools in evaluating the sustainability of increased harvesting
of forest biomass and assessments of critical loads of acidity. A variety of
methods that differ in concept, temporal and spatial scale, and data
requirements are available for measuring weathering rates. In this study,
causes of discrepancies in weathering rates between methods were analysed
and were classified as being either conceptual (inevitable) or random. The
release rates of base cations (BCs; Ca, Mg, K, Na) by weathering were
estimated in podzolised 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 by the depletion method, using Zr as the assumed inert reference; (ii) steady-state weathering rate estimated with the PROFILE model, based on
quantitative analysis of soil mineralogy; and (iii) BC budget 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 horizon
historical weathering of BCs was 10.6 and 34.1 mmolc m−2 yr−1, at Asa and Flakaliden, respectively. Corresponding values of
PROFILE weathering rates were 37.1 and 42.7 mmolc m−2 yr−1.
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 horizons, particularly at Flakaliden. With the exception of Mg and Ca in shallow soil horizons, PROFILE produced
higher weathering rates than the depletion method, particularly of K and Na
in deeper soil horizons. The lower weathering rates of the depletion method
were partly explained by natural and anthropogenic variability in Zr
gradients. The base cation budget approach produced significantly higher
weathering rates of BCs, 134.6 mmolc m−2 yr−1 at Asa and
73.2 mmolc m−2 yr−1 at Flakaliden, due to high rates
estimated for the nutrient elements Ca, Mg and K, whereas weathering rates
were lower and similar to those for the depletion method (6.6 and 2.2 mmolc m−2 yr−1 at Asa and Flakaliden). The large discrepancy in
weathering rates for Ca, Mg and K between the base cation budget approach
and the other methods suggests additional sources for tree uptake in the soil
not captured by measurements.
Current, steady-state and historical weathering rates of base cations at two forest sites in northern and southern Sweden: a comparison of three methods