Seasonal sulfate deposition and export patterns for a small Appalachian watershed

Sulfate deposition and exports from 1988-92 were analyzed for a small headw ater catchment in north-central West Virginia. Annual sulfate inputs, estim ated by applying throughfall-adjusted ratios to bulk deposition values, and outputs were approximately equal for the five years. Annual mean throughfa ll-adjusted deposition and export loads were 55.78 and 55.48 kg ha(-1),resp ectively. While these results indicate the watershed has reached sulfate eq uilibrium relative to current deposition levels, seasonal sulfate accumulat ions and deficits were evident. Deposition and exports averaged 5.61 and 2. 49 kg ha(-1) mo(-1), respectively, during the growing season, and 3.69 and 5.22 kg ha(-1) mo(-1) during the dormant season. Sulfur accumulated within the soil during the growing season mo because inputs of wet and dry sulfur deposition were high while outputs were negligible. The latter was due larg ely to the lack of runoff resulting from high evapotranspirational demands. By contrast, net sulfate losses occurred during dormant seasons, primarily due to high runoff, even though inputs declined during this season. Resear chers working on other watersheds have interpreted similar input/output pat terns to mean that sulfate accumulated during the growing season is the sou rce of sulfate exported during the dormant season. However, radioisotopic e vidence from a companion study on this watershed showed that some labeled s ulfate applied to the watershed more than a year earlier was still present in the organic and mineral soil layers at the point of application (with so me as soluble sulfate), and in soil water dispersed throughout the watershe d. Tts presence indicates that dormant season exports can originate from su lfate deposited over longer periods than just the previous growing season o r even previous year. Volume-weighted concentrations in soil leachate sugge st that dormant-season sulfate losses resulted from progressive depletion o f the anion through the soil profile. During the fall and early winter, sol uble sulfate was depleted in the upper soil horizons; in later winter, depl etion occurred in the lower horizons.