Distribution and uptake of 137Cs in relation to alkali metals in a perhumid montane forest ecosystem

We determined the content of radiocesium (137Cs) and alkali metals in soils, plants (2 ferns, a shrub and moss) and rainwater collected in an undisturbed forest ecosystem. The 137Cs activity and the isotopic ratio of 137Cs/Cs in the samples were used to interpret the distribution and uptake of 137Cs and the alkali metals in plants. As a whole, the 137Cs in plants was assimilated together with K but was not dependent on Cs. Different adaptations of fern species collected in ecological niches cause them to have different 137Cs/Cs ratios. Diplopterygium glaucum is distributed at the edges of the forest; it usually has shallow organic layers, and the root takes up more stable Cs from mineral layers, leading to lower 137Cs/Cs ratios than that in the understory Plagiogyria formosana and Rhododendron formosanum species. The steady supply of stable Cs through the uptake by D. glaucum from deep soils may gradually dilute the 137Cs concentration and thus explain the lower 137Cs/Cs ratio in the fern samples. The 137Cs is predicted to be proportional to the Cs content across plant species in the biological cycle once isotopic equilibrium is attained.