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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

4.1. 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.

Table I
Elemental analysis of soils and U isotopic analysis of soil extracts from selected intervals, MAERC
Site Depth
(cm)
LOI
(%)
Total C
(%)
Organic C
(%)
Inorganic C
(%)
Total N
(%)
Total P
(ppm)
Total S
(%)
34S
(per mil)
Total U
(ppm)
Uranium
extracted
(%)
234U/238U
activity ratio
of extract
S5 0-3 38.7 19.9 16.9 3.0 1.320 602 0.260 16.4 1.4    
3-6 32.7 15.0 15.0 < 0.1 1.030 525 0.212 15.0 1.4 42 1.039
6-9 21.3 11.6 11.2 0.4 0.832 332 0.167 11.9 1.2    
9-12 14.0 7.97 7.54 0.43 0.575 194 0.121 12.5 1.0 21 1.050
12-15 15.3 8.66 8.22 0.40 0.585 184 0.128 14.4 0.9    
15-18 8.0 4.29 4.33 < 0.1 0.337 83.0 0.066 15.0 1.0 10 1.098
18-21 2.9 1.67 1.63 0.04 0.156 41.6 0.023 14.3 0.7    
21-24 2.3 1.36 1.29 0.07 0.135 30.3 0.016 14.2 0.8 5 1.073
24-27 2.1 1.28 1.31 < 0.1 0.141 33.6 0.014 13.1 0.9    
27-30 1.3 0.71 0.71 < 0.1 0.111 31.3 0.003 n.d. 0.7    
770 0-3 52.3 30.5 27.6 2.9 1.840 610 0.348 20.5 0.6    
3-6 22.7 13.8 13.5 0.3 0.969 342 0.198 16.1 0.8 37 1.050
6-9 25.8 13.7 13.7 < 0.1 0.886 218 0.206 18.9 0.3    
9-12 13.5 8.10 7.34 0.76 0.603 120 0.114 20.4 0.5 16 1.108
12-15 3.6 2.35 2.09 0.26 0.184 25.8 0.035 18.5 0.4    
15-18 4.9 3.25 2.78 0.47 0.251 36.6 0.055 20.1 0.5 9 1.162
18-21 1.7 1.03 1.00 0.03 0.087 15.9 0.025 15.4 0.3    
21-24 1.0 0.55 0.54 0.10 0.050 13.8 0.013 20.2 0.3    
24-27 0.4 0.16 0.19 < 0.1 0.020 8.45 0.006 n.d. 0.3    
27-30 0.2 n.d. n.d. n.d. n.d. 5.92 n.d. n.d. 0.3    
Lykes 0-3 14.6 6.86 n.d. n.d. 0.542 206 0.052 n.d. 0.6    
3-6 8.8 5.37 4.39 0.98 0.477 126 0.041 13.1 0.6 7.5 1.109
6-9 7.6 3.70 3.61 0.09 0.340 102 0.027 13.5 0.6    
9-12 5.2 2.71 2.54 0.17 0.253 62.6 0.012 n.d. 0.5 6 1.132
12-15 5.0 2.75 2.29 0.46 0.245 49.5 0.018 n.d. 0.5    
15-18 5.8 2.97 2.71 0.26 0.337 53.6 0.014 n.d. 0.6 6 1.123
18-22 4.4 2.18 2.05 0.13 0.242 41.3 0.012 n.d. 0.8    
n.d.: not determined.


Graphs showing concentrations of total phosphorus, nitrogen, sulfur, organic carbon, and uranium in soil profiles from improved pasture, unimproved semi-native pasture, and native grassland
Figure 2. Graphs showing concentrations of total phosphorus, nitrogen, sulfur, organic carbon, and uranium in soil profiles from improved pasture (S5), unimproved semi-native pasture (770), and native grassland (Lykes). Representative 1-sigma error bars are indicated. [larger image]

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