169
Although we decided to use a single value for init U/Th ratio in this study, additional information on the U/Th ratios in nonagricultural soils was provided by comparing to those of their bedrock data. Table 2 summarizes U/Th data in some
Japanese bedrock (Tezuka 1972; Ebihara et al. 1984; Katoh et al. 1985; Shibata and
Nakamura 1997; Kimura et al. 2003) including rock reference materials prepared by
the National Institute of Advanced Industrial Science and Technology (AIST 2018).
In geochemistry, use of fractionation of U from Th in rocks has been applied successfully to understand the geological conditions of the rocks. The U/Th concentration ratios differ by types of rock due to their generation and aging processes;
volcanic and plutonic bedrocks ranged from 0.15 to 0.51, and sedimentary bedrocks
showed a large variation from 0.25 to 61. As Takeda et al. (2006) reported, it was
clear that the U/Th concentration ratio in a soil reflected that in its bedrock; therefore, to have a more precise ess U datum in an agricultural soil, init U/Th datum in its
bedrock is necessary, but collecting such datum is usually impossible. In the same
table, we list U/Th values for soils that had known bedrock types (Minato 2005).
Because U is slightly more mobile than Th is (e.g., Tomé et al. 2002), the U/Th
values tended to be smaller in nonagricultural field soils than that in bedrocks.
2.2 Concentrations of Estimated ess U in Agricultural Fields
Agricultural soil samples were collected throughout Japan; in agricultural fields,
total U concentrations were from 1.5 to 10.6 mg kg
−1
-dry for 98 paddy field soil
samples and 0.31 to 4.9 mg kg
−1
-dry for 139 upland field soil samples. These data
were measured by ICP-MS after dissolving the powdered soil samples using mineral acid in a microwave oven. Details of the method were reported elsewhere
(Uchida et al. 2007a). Total U concentrations in both uses are distributed lognormally (Fig. 5); therefore, logarithms of the data were used to compare concentrations among different uses. The results showed that total U concentrations were
higher in paddy fields than in upland fields by t-test (p < 0.01). For upland fields, the
soils are generally oxic; therefore, U(VI) is the dominant chemical state and it is
relatively mobile in the soil. However, in paddy fields, because the soils are under
reducing conditions during rice planting, U would exist in the forms of insoluble
U(IV) minerals. Once U(IV) is generated in soil, its insoluble forms can be remobilized due to formation of carbonates from microbial respiration (Wan et al. 2005;
Zhou and Gu 2005). Therefore, the U fates in soil under reducing conditions are
complex, and uranyl carbonates are the most stable.
Uranium from phosphate fertilizer, ess U, has been added to the agricultural fields
for a long time. Therefore, ess U would have partially leached to a deeper layer of a
soil or been removed by drainage water. Interestingly, Takeda et al. (2006) reported
that ess U was retained in the surface layer of the upland field soils. Thus, even under
oxic conditions, ess U can be found in the soil surface layer, that is, in the crop root
zone. For the case of Th, the concentration might also slightly increase due to fertilizer application although the increment would be smaller than that of ess U. Takeda
Soil-to-Crop Transfer Factor: Consideration on Excess Uranium…
Although we decided to use a single value for init U/Th ratio in this study, additional information on the U/Th ratios in nonagricultural soils was provided by comparing to those of their bedrock data. Table 2 summarizes U/Th data in some
Japanese bedrock (Tezuka 1972; Ebihara et al. 1984; Katoh et al. 1985; Shibata and
Nakamura 1997; Kimura et al. 2003) including rock reference materials prepared by
the National Institute of Advanced Industrial Science and Technology (AIST 2018).
In geochemistry, use of fractionation of U from Th in rocks has been applied successfully to understand the geological conditions of the rocks. The U/Th concentration ratios differ by types of rock due to their generation and aging processes;
volcanic and plutonic bedrocks ranged from 0.15 to 0.51, and sedimentary bedrocks
showed a large variation from 0.25 to 61. As Takeda et al. (2006) reported, it was
clear that the U/Th concentration ratio in a soil reflected that in its bedrock; therefore, to have a more precise ess U datum in an agricultural soil, init U/Th datum in its
bedrock is necessary, but collecting such datum is usually impossible. In the same
table, we list U/Th values for soils that had known bedrock types (Minato 2005).
Because U is slightly more mobile than Th is (e.g., Tomé et al. 2002), the U/Th
values tended to be smaller in nonagricultural field soils than that in bedrocks.
2.2 Concentrations of Estimated ess U in Agricultural Fields
Agricultural soil samples were collected throughout Japan; in agricultural fields,
total U concentrations were from 1.5 to 10.6 mg kg
−1
-dry for 98 paddy field soil
samples and 0.31 to 4.9 mg kg
−1
-dry for 139 upland field soil samples. These data
were measured by ICP-MS after dissolving the powdered soil samples using mineral acid in a microwave oven. Details of the method were reported elsewhere
(Uchida et al. 2007a). Total U concentrations in both uses are distributed lognormally (Fig. 5); therefore, logarithms of the data were used to compare concentrations among different uses. The results showed that total U concentrations were
higher in paddy fields than in upland fields by t-test (p < 0.01). For upland fields, the
soils are generally oxic; therefore, U(VI) is the dominant chemical state and it is
relatively mobile in the soil. However, in paddy fields, because the soils are under
reducing conditions during rice planting, U would exist in the forms of insoluble
U(IV) minerals. Once U(IV) is generated in soil, its insoluble forms can be remobilized due to formation of carbonates from microbial respiration (Wan et al. 2005;
Zhou and Gu 2005). Therefore, the U fates in soil under reducing conditions are
complex, and uranyl carbonates are the most stable.
Uranium from phosphate fertilizer, ess U, has been added to the agricultural fields
for a long time. Therefore, ess U would have partially leached to a deeper layer of a
soil or been removed by drainage water. Interestingly, Takeda et al. (2006) reported
that ess U was retained in the surface layer of the upland field soils. Thus, even under
oxic conditions, ess U can be found in the soil surface layer, that is, in the crop root
zone. For the case of Th, the concentration might also slightly increase due to fertilizer application although the increment would be smaller than that of ess U. Takeda
Soil-to-Crop Transfer Factor: Consideration on Excess Uranium…