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

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 513C values in crops can be calculated using the 513C value, —8 %o in air [5], and the 13C fractionation ratio in photosynthesis by rice plants of —18 to —20 %o [6, 7]. The calculated 513C values in our study ranged from —28 %o to —26 %o, 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.

photosynthesis

14C02 gas

Dissolution

Emission

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