Radiocesium in fallen leaves controlled by leaching and associated soil particles: litterbag experiments in Fukushima forests

The contamination of forest ecosystems by radiocesium ( 137 Cs) released from the Fukushima Dai-ichi Nuclear Power Plant accident has seriously damaged forestry activities in Fukushima. Previous field-based descriptive studies have reported that a considerable proportion of 137 Cs remains in the soil O horizon, where the presence of various types of organic matter at different stages of decomposition contributes to both strong retention of 137 Cs by decomposing litter and a heterogeneous spatial distribution. To investigate the dynamics and mobility of 137 Cs during litter decomposition and to interpret our observations, we conducted litterbag experiments over approximately two years using fallen leaves of konara oak. Litterbags were deployed either suspended above the ground or placed on the forest floor. Leaching experiments were also performed on retrieved litterbag samples. In the suspended litterbags, we observed a decline of 137 Cs, mainly in the water-soluble form, likely due to washout associated with rain/snowfall and subsequent spring snowmelt. In contrast, the incorporation of soil particles by forest-floor litterbags resulted in an increase of 137 Cs relative to initial conditions, mainly in forms that were scarcely extractable, even by ammonium acetate. The large variability of residual 137 Cs revealed by comparisons with previous litterbag and equivalent experimental studies highlighted the difficulty of predicting 137 Cs activity concentrations and distributions on the forest floor. Furthermore, comparison with potassium (K), a competing alkali metal, demonstrated that 137 Cs is less mobile than K, regardless of soil particle incorporation. This low mobility likely contributes to the prolonged, strong retention of 137 Cs in the O horizon.