Gypsum application reduces selenium uptake by vegetation on a coal ash landfill

Selenium, a potentially toxic trace element, is present in coal fly ash and is accumulated by plants growing on ash landfills. Application of gypsum (CaSO₄·2H₂O) can reduce Se accumulation. The persistence of this effect and the efficacy of repeated gypsum applications were investigated in forbs and grasses on a soil-capped, fly ash landfill near Lansing, New York. Gypsum was applied as a top-dressing at a rate of 0 or 11.2 t ha^-1 in 1990 to three plots, and one-half of each of these plots received an additional top dressing of gypsum at 11.2 t ha^-1 in 1991, producing four treatments - no gypsum, gypsum only in 1990, gypsum only in 1991, and gypsum in both years. Vegetation was harvested in July and October of 1991 and in July of 1992 and analyzed for Se, S, and Ca. Application of gypsum: (1) decreased Se accumulation by forbs and grasses harvested later in the season; (2) decreased Se accumulation in a subsequent year; (3) produced no further decrease when reapplied in a second year; and (4) did not decrease Se accumulation two years after a single application. The decrease in Se accumulation was not due to dilution caused by S fertilization. Variation in Se accumulation was not explained by variation in the pH and Se concentration of the soil and ash.