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publications > paper > fertilizer-derived uranium and sulfur in rangeland soil and runoff: a case study in central Florida > results and discussion > soil composition
4. Results and Discussion
4.1. SOIL COMPOSITION
Despite very uniform mineralogy of these quartz sands, the soil profiles show variable enrichment of most measured constituents in the upper 10-15 cm compared to values in the deeper intervals (Table I; Figure 2). The profile from the Lykes site (native grassland) shows a maximum near-surface enrichment of 2X to 4X in C, N, P, and S, whereas profiles at sites S5 and 770 from MAERC show near-surface enrichments that commonly exceed 10X. The Lykes profile shows no apparent surface enrichment of U, but stronger indications of surface enrichments are apparent in the other two profiles. Organic matter is most highly concentrated in the uppermost intervals and is the probable host for N, S, P, as well as trace elements such as U that have a strong affinity for organic matter. In contrast, quartz, the major mineral constituent of the soils, has little affinity to accommodate these elements in its structure, but provides surfaces for sorption or for attachment of coatings of organic matter.
Plots of organic carbon, P, N, S and U versus depth (Figure 2) illustrate the similarity in distributions of these constituents in the three profiles. The upper 15 cm of profile S5 (improved pasture) and profile 770 (semi-native pasture) have comparable average OC contents of 11.8 and 13.7 percent, respectively, and also comparable average concentrations of total N and total S. Profile S5 has considerably higher average P content (367 vs. 263 ppm) and higher average U content (1.2 vs. 0.5 ppm) than profile 770. Elevated and highly correlated concentrations of U and P (R2 = 0.94) in uppermost intervals of historically fertilized S5 soil may indicate some contribution from applied fertilizer. Note that concentrations of U in soils of this study (Table I) are low compared to a geometric mean of 2.1 ppm for soils of the conterminous United States (Shacklette and Boerngen, 1984).
The native grassland profile (Lykes) has consistently low total S contents of 0.012 - 0.052 weight percent (Table I), whereas the upper 10-15 cm of the other two profiles (S5, 770) have considerably higher total S contents in the range of 0.1- 0.35 weight percent. Similar near-surface enrichments of S in the fertilized (S5) and unfertilized (770) pasture sites (Figure 2) argue against a significant enrichment of S via fertilization.
U.S. Department of the Interior, U.S. Geological Survey
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