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4. Results and Discussion

Abstract
Introduction
Site Desc. & Land Use
Methods
Results & Discussion
- Soil Composition
- Uranium in Soils
> Sulfur in Soils
- Runoff
- U, S, and P Mobility
Conclusions
Acknowledgments
References
Figures, Tables, & Equations

4.3. SULFUR ISOTOPIC COMPOSITION OF SOILS

Total S in the three soil profiles has delta34S of 11.9 to 20.5parts per thousand symbol (Table I). These values span the same range as most previously published S isotopic data from peat and organic soils in variably impacted areas of the northern Everglades and the highly developed Everglades Agricultural Area (EAA) (Bates et al., 2001). Data for the combined three soil profiles or for each individual profile show no obvious relation between S isotopic composition (delta34S) and depth (Figure 5a), or between S isotopic composition and total S (Figure 5b). The semi-native pasture (site 770) accounts for all of the highest values of delta34S (delta34S = 15.4 to 20.5parts per thousand symbol), whereas the two determinations from the pristine native grassland site (Lykes) are among the lowest values (delta34S = 13.1 and 13.5parts per thousand symbol). delta34S values for the improved and fertilized S5 site are both higher and lower than values from the native grassland site (Lykes), with only a slightly higher average value (14.1parts per thousand symbol versus 13.3parts per thousand symbol).

Graphs showing the sulfur isotopic composition of soils from the three soil profiles with respect to (a) depth and (b) total sulfur content
Figure 5. Graphs showing the sulfur isotopic composition of soils from the three soil profiles with respect to (a) depth and (b) total S content. Representative error bars indicate a 1-sigma analytical precision. The sulfur isotopic composition of locally applied ammonium sulfate fertilizer (3.5parts per thousand symbol) plots well off these graphs. [larger image]
The delta34S value of a sample of locally applied ammonium sulfate fertilizer was 3.5parts per thousand symbol (A. Bates, U.S. Geological Survey, written communication, 2005). Addition of soluble sulfate with this distinctly lighter isotopic composition would lower the average delta34S value in the S5 soil profile compared to the average value of 18.8parts per thousand symbol in the unfertilized soil profile 770 (Figure 5). If S5 received significant ammonium sulfate, the absence of relatively high total S concentrations (Figure 5b) could be explained by more aggressive leaching of S5 soil under conditions of an artificially lowered water table. This interpretation remains problematic, however, because the uppermost layers of S5 do not show lower S-isotopic values caused by addition of isotopically light sulfate from fertilizer (Figure 5a).

Note that bacterial sulfate reduction can severely fractionate S isotopes. A minor role for sulfate reduction is assumed in the above discussion that considers only mixing as a control on sulfur isotope composition in uppermost surface soil layers (0-30 cm). Sulfate reduction requires anoxic conditions that are unlikely to prevail in these dry or intermittently wet surface layers. Bacterial sulfate reduction favors the lighter 32S isotope and so the remaining sulfate becomes isotopically heavier (increased delta34S).

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