Assessing the quasi-equilibrium distribution of Fukushima-derived radiocesium in a typical Japanese cedar forest using an isotopic approach

In this study we applied an isotopic approach to assess a quasi-equilibrium of Fukushima-derived ¹³⁷Cs in a typical mature Japanese cedar forest. As of 2020, the concentrations of ¹³⁷Cs in the aboveground biomass compartments (young sapwood, rest of sapwood, heartwood, inner bark, outer bark, big branches, small branches, young foliage, and old foliage) correlated well with the equilibrium concentrations of the naturally occurring stable isotope of cesium (¹³³Cs) transferred to the aboveground biomass from the soil, indicating that a quasi-equilibrium distribution of ¹³⁷Cs has been achieved after a period of removal of the initially intercepted radiocesium from the biomass compartments. The isotope concentration ratio ¹³⁷Cs/¹³³Cs in the biomass as of 2020 is estimated as 3.6⋅10⁻⁵ (μg kg⁻¹) (μg kg⁻¹)⁻¹ (95 % confidence interval (CI): 1.6⋅10⁻⁵ – 5.6⋅10⁻⁵). The isotope inventory ratio ¹³⁷Cs/¹³³Cs in the 5-cm topsoil is slightly lower, (2.52 ± 2.39)⋅10⁻⁵ (μg kg⁻¹) (μg kg⁻¹)⁻¹, which may indicate the presence of some amounts of ¹³⁷Cs intercepted from the atmospheric fallout in March 2011 in the biomass compartments and the predominant uptake of Cs isotopes from the topsoil layer thinner than 5 cm. The total ¹³⁷Cs inventory in aboveground biomass stabilized between 2017 and 2020 at the level of (3.1 ± 1.7)% of its total inventory in the ecosystem.