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publications > paper > fertilizer-derived uranium and sulfur in rangeland soil and runoff: a case study in central Florida > introduction
Possible sources of P and nitrogen (N) in ranchland runoff include fertilizer, manure, and local organic soil. The mobility of P and N from these sources is enhanced by drainage improvements, soil disturbance, or fertilizer-enhanced bacterial activity. Determining contributions of nutrients from multiple sources is complicated and in most cases involves controlled field studies, extensive multi-scale sampling, and mass balance calculations tied to the type and intensity of land use (Fluck et al., 1992; Halberg and Keeney, 1993; Spalding and Exner, 1993; Boggess et al., 1995). In some cases, nitrogen isotope ratios can be used to distinguish between isotopically distinct nitrogen sources from fertilizer or animal waste, but interpretations are generally complicated by bacterially-mediated processes of nitrification and denitrification that can modify nitrogen isotopic compositions (Heaton, 1986; Seiler, 2005). Similar isotopic identification of P sources is not possible because natural P is monoisotopic.
Soil and water samples for this study were collected within the south half of the 4,171-ha Buck Island Ranch, Highlands County, central Florida (Figure 1). This fully operational cattle ranch is the main component of the MacArthur Agro-Ecology Research Center (MAERC), a division of Archbold Biological Station. The MAERC was established in 1988 as a center for research on the relation between cattle ranching and the native ecosystems of central Florida. In particular, MAERC provides opportunities for controlled, field-scale experiments that monitor the effects of various ranchland management practices on soils, runoff, and the local wetland and grassland ecosystems (Capece et al., 2006; Swain et al., 2006).
In this study, U concentrations and U isotopic compositions in collected samples are used to identify, and in some cases quantify, fertilizer-derived U in soil and surface runoff. Data from a soil profile in improved pasture, with a history of P fertilizer application are compared and contrasted with similar data from nearby semi-native pasture and undeveloped native grassland, with no history of P fertilizer application. Surface water runoff samples from the improved pasture are also compared to contemporaneous runoff samples from unimproved pasture. Finally, the mode of occurrence and mobility of U and S in soil and water are evaluated with respect to possible analogous behavior of P and N.
U.S. Department of the Interior, U.S. Geological Survey
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