Isotope studies to the sorption behavior of atmospheric sulfate in humus layers of scots pine ecosystems

The sorption potential for SO4(2-) in humus layer samples from field sites along a deposition gradient was determined experimentally in batch experiments. The Freundlich equation was used to quantify the sorption of added SO4(2-) in humus layer samples and to determine site-dependent sorption parameters. SO4(2-) sorption in humus layers is a concentration-dependent process. The linearity of isotherms reveals that SO4(2-) is reversibly bound in the organic surface layer, as long as soil solution concentrations remain above 26 to 44 mg SO4(2-) L(-1). Natural isotope variations of sulfur in SO4(2-) were analysed to investigate the degree of sorption of dissolved atmospheric and added SO4(2-). Both sulfate species differed significantly in their isotope composition. The pattern of delta34S values for SO4(2-) in all equilibrium solutions confirm the findings from sorption isotherms, showing a close relationship between the sulfur isotope ratios of SO4(2-) in soil solutions and the amount of SO4(2-) sorbed at the humus layer matrix. Stored atmospheric SO4(2-) in humus layers is released at sites where sulfate concentration in throughfall drops below 26 mg SO4(2-) L(-1). Concentration of soluble Fe decreased with increasing sulfate sorption, thus supporting the assumption that active Fe for example is important. Iron probably stabilizes the reactive surface of humus complexes and therefore has a positive influence on the SO4(2-) sorption in humus layers.