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5. Conclusions

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Historic application to improved pasture (S5) of P-based fertilizers containing trace amounts of U is indicated by elevated concentrations of P and U in the uppermost few centimeters of soil compared to deeper levels, or as compared to uppermost soils at an undeveloped native grassland site (Lykes). Additional evidence of fertilizer-derived U is provided by a distinctive, fertilizer-like isotopic composition of U in soil extracts. Based on isotope and mass balance calculations for soil extracts, the weight fraction of fertilizer-derived U in historically fertilized soil S5 can be estimated as 15-34 percent. Fertilizer-derived U and natural U are efficiently sorbed onto soil organic matter and this results in very low concentrations ( < 0.1 ppb) of dissolved U in runoff water. Runoff from fertilized pasture (S5) has a U isotopic composition consistent with some contributions from a fertilizer source.

Despite having a distinctive isotopic signature, S derived from ammonium sulfate fertilizer is difficult to detect in soil profiles or runoff because other sources of S are relatively large and vary in S concentration and S isotope composition. Concentrations of dissolved sulfate in S5 runoff are elevated compared to runoff from unimproved pasture W4, primarily because drainage improvements and land use practices at S5 increase the leaching efficiency of sulfate.

Organic-bearing soils at the study site efficiently retain historically added U, and by inference, also retain at least some added P. Efforts to limit P in runoff from similar soils in central Florida are well served by management techniques that preserve soil organic matter and that minimize transport of suspended (colloidal or particulate) organic matter in runoff.

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U.S. Department of the Interior, U.S. Geological Survey
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