Weathering release rates of base cations from soils within a boreal forested catchment: variation and comparison to deposition, litterfall and leaching fluxes

Abstract

Long-term (10,200 years) average annual weathering rates (mass losses) of base cations (Ca, Mg, and K) for ten podzolic soil profiles in a remote, forested catchment in eastern Finland were determined using the Zr-depletion method. Three of the profiles were developed in sorted glaciofluvial parent material (sand overlying till) and seven developed in stony till parent material. The aims were to determine the variation in base cation weathering rates within the catchment; to determine if weathering rates differed between the two types of parent material; and to compare the weathering rates with other ecosystem fluxes of base cations, namely total deposition (TD), litterfall (LF), and soil leaching (40 cm). Soil and parent material were sampled to a depth of 3.5 m and total elemental concentrations in the <2 mm fraction determined using XRF. Annual mean (1989–1997) TD, LF, and leaching fluxes have been determined at two of the plots as part of long-term ecosystem monitoring programmes. There was no significant (p < 0.05) difference in Ca + Mg + K weathering rates between the two deposit types, with rates averaging 15.3 and 14.9 mmolc m−2 year−1 for glaciofluvial sand and till profiles, respectively. The coefficient of variation in Ca + Mg + K weathering rates across all ten profiles was 31 %. The weathering rates were similar in size to the LF and leaching fluxes of base cations, and were also sufficient to neutralize the TD load of SO4 2− acidity. Our results indicate that soil weathering is an important source of base cations and essential to the sustainability of site productivity of boreal catchments receiving low deposition loads, even when having soil parent material dominated by acidic silicate minerals.