Transfer of 14C from Soil to Rice Plants

Soil-to-rice plant transfer factors (TFs) of 14C, which was defined as 14C concentration in rice grains (Bq/kg-dry) divided by that in soil (Bq/kg-dry), were determined by laboratory and field experiments. In the laboratory experiment using a

Table 26.1 The partitioning ratios of 14C into solid, liquid, and gas phases for each treatment.


Partitioning ratio (%)

Solid phase

Liquid phase

Gas phase









Glutaraldehyde exposure




Cycloheximide exposure




Fig. 26.4 Colonies of bacteria (a) and their autoradiography image (b). Heterotrophic bacteria have the ability to uptake 14C from an agar medium

growth chamber, we grew rice plants with addition of [1,2-14C] sodium acetate. This 14C compound was supplied once to rice plants in the flooding water just before blooming, and TF of 6.8 ± 2.4 on average was obtained. In these tracer experiments, rice plants were also cultivated without [1,2-14C] sodium acetate as negative controls in the same growth chamber as the 14C-treated rice. Interestingly 14C was detected even from the rice grains of negative control samples. These results suggested that the 14C-bearing gas, which was released from bacterial cells in rice paddy soils, was fixed by the rice plants in the negative controls through photosynthesis.

We also examined the possibility of root uptake of 14C by stable isotope techniques under field conditions [4]. If plant carbon originates from the atmospheric CO2, the δ13C values in crops can be calculated using the δ13C value, -8 ‰ in air [5], and the 13C fractionation ratio in photosynthesis by rice plants of -18 to -20 ‰ [6, 7]. The calculated δ13C values in our study ranged from -28 ‰ to -26 ‰, and the results implied that no soil carbon contribution occurred for white rice; however, by setting some conditions, for example, 13C fractionation ratio of 19‰, we obtained the average TF value of 0.11 ± 0.04 for white rice. To compare these TF values obtained in laboratory and field experiments, it is necessary to pay attention to the difference between [1,2-14C] sodium acetate and the actual organic compounds present in the natural soil.

Fig. 26.5 A conceptual diagram for the behavior of 14C in rice paddy fields

Behavior of 14C in Rice Paddy Fields

From the aforementioned results, the behavior of 14C in rice paddy fields could be considered as follows (a conceptual diagram appears in Fig. 26.5). When irrigation water is contaminated by 14C-bearing sodium acetate, the 14C compound is taken up and metabolized by indigenous bacteria. A part of the 14C is assimilated by the bacterial cells, and the rest of the 14C is released as gaseous compounds from the cells as a result of dissimilation. The dominant chemical species of 14C in gas forms is carbon dioxide, and thus some of the released 14CO2 is dissolved in soil solution depending on pH. For example, when the pH of the soil solution is less than 6.5, most of 14C in gas forms is released into the air. The released 14CO2 is eventually taken up by rice plants during photosynthesis. When the pH of the soil solution is between 6.5 and 10.5, 14C-bearing bicarbonate ion dominates in the soil solution. In addition, once 14CO2 has been released into the air, a part of the 14CO2 gas may be redissolved in the soil solution again as bicarbonate ion. When the pH of the soil solution is greater than 10.5, although this is not probable in paddy fields, 14Cbearing carbonate ion dominates in the soil solution. Carbonate ion is thermally unstable and thus precipitates as carbonate minerals such as CaCO3. In these alkaline situations, the ratio of 14C in the solid phase may increase as a result of the precipitation of 14C. Because the root uptake of 14C by rice plants is negligible, gasification of 14C is an important environmental transfer pathway for the safety assessment of TRU wastes, and bacteria are responsible for driving this pathway.

Acknowledgments This work has been partially supported by the Agency for Natural Resources and Energy, the Ministry of Economy, Trade and Industry (METI), Japan.

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