Abstract: Changes in sulfur constituents and 35S-activity of soil horizons during incubation and after 35S-sulfate addition were used to estimate sulfur flux rates in an Adirondack hardwood forest soil.
Net release of sulfate occurred in the Oa, Bh, Bsl, and Bs2 horizons during biweekly leaching with dionized water. Mineralization rates were 4.1, 0.7, 2.5 and 0.8% of total sulfur over 20 weeks, respectively. The source of mineralized sulfur was carbon-bonded S in the Oa and Bh horizons, and ester sulfate in the Bsl and Bs2 horizons.
Incorporation of 35S-labeled sulfate into organic and inorganic sulfur occurred during incubation of the Oa and Bsl horizons. In the Oa horizon transformation of 35S-sulfate into carbon-bonded S accounted for 62% of the 35S-activity after 56 days. In the Bsl horizon adsorption of 35S-sulfate constituted 54% of the added radioisotope after 56 days. Desorption and subsequent transformation of 35S-sulfate contributed to 35S incorporation into organic forms in the Bsl horizon.
Leaching of horizons with 35S-labeled soil solution was used to measure sulfur fluxes under simulated field conditions. Net release of sulfate occurred during leaching of the Oie and Oa horizons. Accumulation of sulfate occurred in the Bsl horizon. Mineralization rates of sulfur were 11.3 and 0.7% of total sulfur over 20 weeks for the Oie and Oa horizons, respectively. The net immobilization rate of sulfate in the Bsl horizon was 2.7% of total S over 20 weeks. all three horizons lost carbon-bonded S through leaching though the decrease was greatest for the organic horizons. Incorporation of 35S-sulfate into organic and inorganic sulfur constituents occurred in all horizons. Only 5% of 35S applied during leaching was incorporated in the Oie and Oa horizons; most if the incorporated 35S was subsequently leached as carbon-bonded-35S. In the Bsl horizon 23% of the 35S-sulfate was incorporated into the soil with adsorption being the dominant process. Results illustrated the importance of organic sulfur mobility and indicated that mineralization-immobilization and adsorption-desorption rates vary among the horizons. These differences are reflected in the vertical distribution of the organic and inorganic sulfur constituents of this spodosol.