Time-dependent changes of phytoavailability of Cs added to allophanic Andosols in laboratory cultivations and extraction tests.

Although it is well known that phytoavailability of radiocaesium is gradually lost after its deposition on the ground by fixation to soil minerals, the decreasing rates during early period after the deposition is not yet quantitatively evaluated. In this study, stable Cs was added to 5 types of soil, including Andosols, a sand-dune regosol and a smectic lowland soil, in a laboratory soil incubation experiment to assess the aging effect of radiocaesium. Aliquots of a soil sample were put into pots and incubated in an artificial climate chamber. Orchardgrass or red clover was cultivated for 28 d in soil pots containing one of the allophanic Andosol samples seven times during about 1200 d using new pots for each cultivation. The soil-to-plant transfer factors of Cs declined exponentially until about 100 d and were almost constant thereafter. The extractabilities of Cs by water and 1 M NH4OAc solution from this allophanic Andosol soil sample also decreased with time and their decreasing patterns were similar to that of the transfer factor. The temporal changes of extractabilities of Cs in other soil samples were also examined 6 times during about 600 d. Rate of decline for the extracted yield of the added Cs by 1 M NH4OAc varied widely among all the soil types. Two allophanic Andosol samples showed relatively higher extractabilities in comparison with the other soils throughout the incubation experiment, which may be attributable to the lower contribution of Cs specific sorption sites to total cation exchange capacity of the allophonic Andosol soil samples.