Weathering release of heavy metals from soil in comparison to deposition, litterfall and leaching fluxes in a remote, boreal coniferous forest

Soil weathering release rates of Cd, Cu, Ni, Pb and Zn were determined at 4 plots in a forested catchment located in eastern Finland. The rates are compared with atmospheric deposition, litterfall and leaching fluxes at two of the plots. The soils were Haplic Podzols and developed in glaciofluvial deposits or glacial till having sandy loam to loamy sand fine-earth (<2 mm) textures. The bedrock in the area consists of granodiorites. Total concentrations of heavy metals in the <2 mm fraction of the parent material were determined from a mixed acid (HF and HClO4) digestion using ICP–MS. Weathering rates were calculated assuming that the molar ratio of heavy metal to base cation (Ca+Mg) weathering rates (previously determined by the Zr depletion method) equalled the molar ratio of heavy metal to base cation concentrations in the parent material. The mean weathering release rates were 8.3 μmol m−2 a−1 for Zn, 5.2 for Ni, 4.3 for Cu, 1.5 for Pb, and 0.011 for Cd. These rates were similar to the deposition, litterfall, and leaching fluxes for each metal. Weathering is therefore an important process in the heavy metal biogeochemistry of boreal, forest ecosystems.