Distribution of radiocesium and its controlling factors under the Japanese cedar canopies

This study investigated the spatial distribution of radiocesium deposited by the Fukushima Daiichi Nuclear Power Plant accident in a densely planted Japanese cedar stand. Systematic grid sampling was conducted to determine ¹³⁷Cs inventories in the layers of deposited organic material and mineral soil at two different spatial scales (hillslope [60 m²] and small [1 m²]). The results showed that ¹³⁷Cs inventories along the hillslope were heterogeneously distributed, with coefficients of variation for the deposited organic material and mineral soil layers of 46.4% and 48.9%, respectively. The ¹³⁷Cs inventory in each layer tended to show a lognormal distribution. The correlation between the ¹³⁷Cs inventories in deposited organic material and mineral soil in the same sampling grid was weak. The controlling mechanisms of the ¹³⁷Cs inventories in the litter and mineral soil layers differed due to differences in the underlying key processes, such as canopy-forest floor transfer due to hydrological and biological processes. No significant correlation was found between the distance from the nearest tree trunk and the ¹³⁷Cs inventory in the deposited organic layer at each sampling point. In contrast, the ¹³⁷Cs inventory in the soil tended to increase as the distance from the nearest tree trunk increased at both the hillslope and small scales. It was found that the initial spatial patterns of ¹³⁷Cs in the soil layer due to atmospheric deposition were preserved in the cedar stand. Finally, we tested the effects of soil sampling density on the reliability of mean soil ¹³⁷Cs inventory estimations in the cedar stand. The results indicated that a soil sampling area greater than 0.06 m² at the hillslope scale and 0.008 m² at the small scale enabled the mean ¹³⁷Cs inventory to be estimated with an uncertainty of less than 20% in the cedar stand.