SULFATE RETENTION AND RELEASE IN SOILS AT PANOLA MOUNTAIN, GEORGIA

Inorganic sulfate pools, sulfate sorption characteristics, and Fe and Al fractions were determined on soils at Panola Mountain, a 41-ha forested watershed in the Georgia piedmont. Sulfate adsorption was measured in batch solutions that bracketed the range of ambient sulfate concentrations and soil solution acidity. The slope and intercept of the initial mass (IM) isotherms, formed from plots of sulfate retained against sulfate added, were used to compare sorption behavior among soils. The reversibility of sulfate adsorption was determined by measuring desorption of soluble sulfate from soil before and after equilibration with a concentrated sulfate solution. Sulfate sorption properties of soils at Panola Mountain fall along a continuum between two end members. The “low-adsorbing” end member comprises shallow soils (0–10 cm), with high water-soluble sulfate (Sw), low phosphate-extractable sulfate (Sp-w), high organic matter, low sulfate retention ability (IM isotherm slope near 0.0), and high sulfate adsorption reversibility. The “high-adsorbing” end member comprises deeper soils (>10 cm), with higher total native sulfate (mostly as Sp-w), low organic matter, high sulfate retention ability (isotherm slope near 1.0), and low sulfate adsorption reversibility. Sulfate retention was only weakly related to Fe and Al fractions, possibly because of inhibition of adsorption by organic matter. Sulfate concentrations in surface waters reflect the spatial distribution of soil sulfate retention properties; baseflow, representing water which has equilibrated with the mineral subsoil, has sulfate concentrations near 10 μeq/L, whereas stormflow, which is dominated by water flowing through shallow horizons, has sulfate concentrations near 100 μeq/L.