26.5 Behavior of
14
C in Rice Paddy Fields
From the aforementioned results, the behavior of
14
C in rice paddy fields could be
considered as follows (a conceptual diagram appears in Fig. 26.5). When irrigation
water is contaminated by
14 C-bearing sodium acetate, the
14 C compound is taken up
and metabolized by indigenous bacteria. A part of the
14 C is assimilated by the
bacterial cells, and the rest of the
14 C is released as gaseous compounds from the
cells as a result of dissimilation. The dominant chemical species of
14 C in gas forms
is carbon dioxide, and thus some of the released
14 CO 2 is dissolved in soil solution
depending on pH. For example, when the pH of the soil solution is less than 6.5,
most of
14 C in gas forms is released into the air. The released
14
CO 2 is eventually
taken up by rice plants during photosynthesis. When the pH of the soil solution is
between 6.5 and 10.5,
14 C-bearing bicarbonate ion dominates in the soil solution. In
addition, once
14 CO 2 has been released into the air, a part of the
14 CO 2 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,
14 Cbearing carbonate ion dominates in the soil solution. Carbonate ion is thermally
unstable and thus precipitates as carbonate minerals such as CaCO 3 . In these
alkaline situations, the ratio of
14 C in the solid phase may increase as a result of
the precipitation of
14
C. Because the root uptake of
14 C by rice plants is negligible,
gasification of
14 C 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.
Open Access This chapter is distributed under the terms of the Creative Commons Attribution
Noncommercial License, which permits any noncommercial use, distribution, and reproduction in
any medium, provided the original author(s) and source are credited.
Fig. 26.5 A conceptual diagram for the behavior of
14
C in rice paddy fields
308
N. Ishii et al.
14
C in Rice Paddy Fields
From the aforementioned results, the behavior of
14
C in rice paddy fields could be
considered as follows (a conceptual diagram appears in Fig. 26.5). When irrigation
water is contaminated by
14 C-bearing sodium acetate, the
14 C compound is taken up
and metabolized by indigenous bacteria. A part of the
14 C is assimilated by the
bacterial cells, and the rest of the
14 C is released as gaseous compounds from the
cells as a result of dissimilation. The dominant chemical species of
14 C in gas forms
is carbon dioxide, and thus some of the released
14 CO 2 is dissolved in soil solution
depending on pH. For example, when the pH of the soil solution is less than 6.5,
most of
14 C in gas forms is released into the air. The released
14
CO 2 is eventually
taken up by rice plants during photosynthesis. When the pH of the soil solution is
between 6.5 and 10.5,
14 C-bearing bicarbonate ion dominates in the soil solution. In
addition, once
14 CO 2 has been released into the air, a part of the
14 CO 2 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,
14 Cbearing carbonate ion dominates in the soil solution. Carbonate ion is thermally
unstable and thus precipitates as carbonate minerals such as CaCO 3 . In these
alkaline situations, the ratio of
14 C in the solid phase may increase as a result of
the precipitation of
14
C. Because the root uptake of
14 C by rice plants is negligible,
gasification of
14 C 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.
Open Access This chapter is distributed under the terms of the Creative Commons Attribution
Noncommercial License, which permits any noncommercial use, distribution, and reproduction in
any medium, provided the original author(s) and source are credited.
Fig. 26.5 A conceptual diagram for the behavior of
14
C in rice paddy fields
308
N. Ishii et al.