Fertilizer-Derived Uranium and Sulfur in Rangeland Soil and Runoff: A Case Study in Central Florida

Abstract

Fertilizer applications to rangeland and pastures in central Florida have potential impact on the nutrient–sensitive ecosystems of Lake Okeechobee and the Northern Everglades. To investigate the effects of fertilizer applications, three soil profiles from variably managed and improved rangeland, and four samples of surface runoff from both fertilized and unfertilized pasture were collected. In addition to determining nutrient concentrations, isotopic analyses of uranium (U) and sulfur (S) were performed to provide isotopic evidence for U derived from historically applied phosphate (P)-bearing fertilizer (234U238U activity ratio =1.0 ± 0.05), and Sderived from recently applied ammonium sulfate fertilizer(δ34S=3.5permil).The distribution and mobility of fertilizer−derived U in these samples is considered to be analogous to that of fertilizer−derived phosphate.Variations of U concentrations and234U/238U activity ratios in soils indicate contribution of fertilizer-derived U in the upper portions of the fertilized soil (15–}34 percent of total U). The U isotope data for runoff from the fertilized field also are consistent with some contribution from fertilizer-derived U. Parallel investigations of S showed no consistent chemical or isotopic evidence for significant fertilizer-derived sulfate in rangeland soil or runoff. Relatively abundant and isotopically variable S present in the local environment hinders detection of fertilizer-derived sulfate. The results indicate a continuing slow-release of fertilizer-derived U and, by inference, P, to the P-sensitive ecosystem, and a relatively rapid release of sulfate of possible natural origin.