Sulfur Isotope Signals in Forest Soils of Central Europe along an Air Pollution Gradient

Stable isotopes were used to study sulfur dynamics in forest soils of the Czech Republic (Central Europe), one of the most polluted regions in the world. δ34S ratios were determined in vertical soil profiles in 18 small forested catchments covering a wide range of pollution levels from 8.8 to 78.8 kg of S ha-1 yr-1. All sites exhibited an increase in δ34S ratios from topsoil to mineral soil, regardless of the distance from major point sources of atmospheric pollution. Of these 18 sites, 15 sites were characterized by a smooth positive isotope signal with the highest δ34S in the deepest level sampled. The remaining three sites had a δ34S maximum in the second deepest horizon. With increasing soil depth, the positive δ34S signals accompanied a negative S concentration gradient. Our findings constrain the relative roles of various mechanisms of sulfur isotope redistribution in the soil depth profile. Even at extremely high atmospheric deposition (>70 kg of S ha-1 yr-1) sulfur appears to be cycled through an organic S pool in the forest soil.