publications > paper > fertilizer-derived uranium and sulfur in rangeland soil and runoff: a case study in central Florida > results and discussion > estimates of fertilizer-derived uranium in soils
4. Results and Discussion
4.2. ESTIMATES OF FERTILIZER-DERIVED U IN SOILSRanchland soils of this study are not subject to tillage, and therefore are more likely to preserve evidence of minor U addition to uppermost soil layers. Fertilizer U stored in these layers could originate from previous direct applications of phosphate fertilizer or be added by runoff originating from historically fertilized upland areas. Another possible source of U could be manure, to the extent that consumed vegetation contains U. Uranium is a nonessential element for plants and is present in low to very low parts-per-billion (ppb) levels in fresh plant tissue (Mortvedt, 1994; Linsalata, 1994). For this reason, native vegetation and vegetation-based nutritional supplements such as molasses should not contain significant U. A 20-fold concentrate of sugar cane leaves produced by ashing contained undetectable U (< 200 ppb) by delayed neutron analysis (Zielinski et al., 2000). In feedlot settings, manure may contain U present in the phosphorus mineral supplements of cattle feeds (Reid et al., 1977). In one study of dairy cattle, virtually all of ingested U was found to pass with the feces (Chapman and Hammons, 1963). For the rangeland cattle at MAERC, contributions of U from ingested plants or artificial feeds are likely to be minimal, eliminating manure as a significant U host. Export of U from improved pasture such as S5 is primarily via dissolution of U that originally resided in soil or fertilizer particles.
Uranium isotopic compositions of loosely-bound, extractable U were used to estimate the amount of fertilizer-derived U in soil profiles. Uranium isotopes are not fractionated during sorption or desorption and thus the AR of soil extracts represents the time-averaged isotopic composition of dissolved U in contact with soil-based sorbants such as organic matter. Extracts from the upper soil layers of S5 have the lowest measured 234U/238U activity ratios (Table I; Figure 3). These low AR values of 1.039 and 1.050 fall within the narrow range of 1.0 ± 0.05 reported for commercial phosphate fertilizers derived from acid treatment of phosphate rock (Zielinski et al., 2000). Isotope-based estimates of the amount of fertilizer-derived U in these extracts involve calculations based on the contrasting isotopic composition of fertilizer U and natural U. An AR of 1.020 ± 0.002 for the fertilizer end member is based on the average value of locally obtained superphosphate products (Table II) that are applied in pure form or as part of fertilizer blends. Also included in Table 2 are AR values for five nutritional mineral supplements to cattle feed that contain variable amounts of U and P. The narrow range of AR values for these supplements (1.015 to 1.033) also falls within the range for phosphate fertilizers and indicates a probable phosphate-rock origin for the P. An AR of 1.127 ± 0.022 for natural U is based on the average of three extracts from the native grassland profile (Lykes) and two extracts from the lower portion of the semi-native profile 770 (Table I). Extractable U from these lower soil layers is assumed to represent U sorbed onto organic matter under natural or pre-fertilization conditions. Extractable fractions of U in the 10 treated soils ranged from 5 to 42 percent and correlate positively with organic matter content (Table I; Figure 4).
ARext. = (F) x (ARfert) + (1 - F) x (ARnat) (1)
where F: mass fraction of U from fertilizer, 1 - F: mass fraction of natural U, AR = 234U/238U activity ratios of fertilizer and natural end members (see above), and the activity ratio of a measured extract.
F = (ARext - ARnat)/(ARfert - ARnat) (2)
Based on this calculation, 82 weight percent of the uranium extracted from the 3-6 cm interval of S5 is fertilizer-derived. Extractable U in this interval was 42 percent of total U (Table I), so 34 weight percent of total U in this interval is calculated to be fertilizer-derived. This estimate is a minimum if the extraction did not yield all readily extractable U or if some fertilizer U is present as less soluble particles. A similar calculation for the 9- to 12-cm section of S5 indicates that 72 weight percent of extractable U and 15 weight percent of total U is fertilizer-derived. The concentration of remaining unextractable U in these uppermost intervals is on the order of 0.8-0.9 ppm, similar to concentrations in organic-poor lower intervals (Table I).
The 3- to 6-cm section of semi-native profile 770 may also contain some fertilizer-derived U, based on the AR of 1.050 in its extract. Minor amounts of fertilizer-derived U could be introduced during flooding or by undocumented applications of fertilizer. Calculations indicate that 72 weight percent of extractable U and therefore 27 weight percent of total U in this upper interval may be fertilizer-derived. Unextractable U concentration is on the order of 0.6 ppm, similar to concentrations in organic-poor lower intervals (Table I).
U.S. Department of the Interior, U.S. Geological Survey
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