Biogeochemistry of aluminum in a forest catchment in the Czech Republic impacted by atmospheric inputs of strong acids

Abstract

The Lysina catchment in the Czech Republic was studied to investigate the biogeochemical response of Al to high loadings of acidic deposition. The catchment supports Norway spruce plantations and is underlain by granite and podzolic soil. Atmospheric deposition to the site was characterized by high H⁺ and SO₄ ²⁻ fluxes in throughfall. The volume-weighted average concentration of total Al (Al_t) was 28 μmol L⁻¹ in the O horizon soil solution. About 50% of Al_t in the O horizon was in the form of potentially-toxic inorganic monomeric Al (Al_i). In the E horizon, Al_t increased to 71 μmol L⁻¹, and Al_i comprised 80% of Al_t. The concentration of Al_t (120 μmol L⁻¹) and the fraction of Al_i (85%) increased in the lower mineral soil due to increases in Al_i and decreases in organic monomeric Al (Al_o). Shallow ground water was less acidic and had lower Al_t concentration (29 μmol L⁻¹). The volume-weighted average concentration of Al_t was extremely high in stream water (60 μmol L⁻¹) with Al_i accounting for about 60% of Al_t. The major species of Al_i in stream water were fluorocomplexes (Al-F) and aquo Al³⁺. Soil solutions in the root zone were undersaturated with respect to all Al-bearing mineral phases. However, stream water exhibited Al_i concentrations close to solubility with jurbanite. Acidic waters and elevated Al concentrations reflected the limited supply of basic cations on the soil exchange complex and slow weathering, which was unable to neutralize atmospheric inputs of strong acids.