S Cycling in soil and stream sediment: influence of season and in situ concentrations of carbon, nitrogen and sulfur

Carbon and nitrogen and sulfur concentrations were measured to determine if associations existed between soil nutrient status, season, and the potential for immobilization of precipitation-borne sulfate in a forest watershed. Radiolabeled sulfate was used to assess the fate of S in soil and sediment. Samples were collected from mineral soils on southerly and northerly slopes at 0.5 m, 5 m, and 15 m from the active channel of a first order stream at Coweeta Hydrologic Laboratory, near Otto, NC. Soils were compared to sediments in their property to buffer against added sulfate, an anion implicated in leaching soil cations and contributing to soil acidity. Two major fates of S were (1) physico-chemical adsorption and (2) incorporation of sulfate-S into organic matter. No differences were seen due to soil sample source, either by season, slope aspect, or position on a slope. Soils adsorbed an average of 48% of added S, while only 23% was adsorbed in sediments. The rate of formation of organic S was also 2-to-3 times higher in soil than sediment. Soil and sediment adsorption potentials were not related to carbon, nitrogen or the C-to-N ratio. Sediment S immobilization was greater that than in soils when compared on a per-unit C basis. Sediments may be an important source of additional S-immobilization in streams that drain S-affected forest ecosystems.