A soil contamination map seems like it should predict how much radiocesium ends up in an edible plant. But for Koshiabura buds, measuring the soil more precisely did not eliminate the variation—and the key may be which soil layer contains the cesium.

The aggregated transfer factor, or Tag, represents the actual transfer of radiocesium from soil to wild edible plants, but its values have shown substantial variation since the Fukushima nuclear accident. This study investigated spatial scale and vertical 137Cs distribution in soil as possible sources of that variation for Eleutherococcus sciadophylloides, or Koshiabura, one of the most severely contaminated wild edible plants.

Directly measuring 137Cs deposition in soil from the habitat did not reduce Tag-137Cs variation compared with spatially averaged airborne-survey data at the administrative-district scale. Publicly available wild-plant concentration data usually lack detailed collection-point information, while measuring 137Cs deposition at every collection point is time-consuming and expensive.

Because of this, Tag-137Cs denominators have been calculated at several spatial scales, including municipalities, smaller administrative districts, and individual points. The plant’s habitat preference can also make spatial averages misleading, because each species may occur within a forest, at a forest margin, or outside a forest.

The study examined two specific factors behind Tag-137Cs variation: spatial scale and the depth dependency of 137Cs deposition in soil. Three spatial scales were used: municipality, administrative district, and sampling point. The study also tested correlations between 137Cs activity in Koshiabura buds and deposition in each soil layer, then used those evaluations to discuss methods for calculating Tag-137Cs for wild edible plants.

Figure six maps Iitate Village’s administrative districts over the fifth MEXT airborne survey’s cesium-one-thirty-seven deposition data. The color scale indicates deposition from five to nine thousand kilobecquerels per square meter, while labeled points mark Koshiabura bud and soil sampling sites, including locations in Sasu, Hiso, Yamakiya, and Mukaiyama.

This spatial context matters because the authors compare contamination estimates at the district and site scales. Table 1 compares Tag-137Cs values for Koshiabura buds across previous studies and the present results, using different ways to obtain soil 137Cs deposition at collection sites.

Table one compares the aggregated transfer factor of cesium one hundred thirty-seven in Koshiabura buds across four spatial scales for deposition data. It reports sample size, geometric mean, geometric standard deviation, and minimum and maximum values: the geometric means range from five point two times ten to the power of minus three to nine point one times ten to the power of minus three square meters per kilogram fresh matter.

The authors use this comparison to examine how spatial scale and local measurement affect the reported variability. Approximate Koshiabura-bud 137Cs concentrations can be estimated from soil 137Cs deposition regardless of spatial scale, because the geometric-mean values stay within the same order of ten to the power of minus three.

Narrowing the spatial scale reduced Tag-137Cs variability: the geometric standard deviation fell from 3.5 at the municipal scale to 2.0 or 2.6 at the administrative-district scale. However, directly measured soil data had similar variability to publicly available administrative-district data, so more accurate total-soil deposition did not eliminate Tag-137Cs variation.

The results therefore suggest that total soil contamination does not necessarily directly reflect radiocesium concentration in Koshiabura buds. Figure Three tracks the annual aggregated transfer factor of cesium-137 from soil into Koshiabura buds, using publicly available district-level data from Iitate Village between 2014 and 2020.

Blue points show individual estimates, while orange circles show geometric means; the measured values from 2019 to 2020 are included for comparison. The wide spread of yearly points shows substantial variability, indicating that similar district-level soil deposition can still correspond to varied cesium concentrations in the buds.

Figure 4 compares bud 137Cs activity concentration with deposition in the organic horizon, the mineral soil horizon from the surface to ten centimeters, and the combined horizons. The regression analysis pooled data from three forest types. Figure four compares cesium-137 activity in Koshiabura buds with cesium-137 deposition in the organic horizon, mineral soil horizon, and both horizons combined.

Using pooled data from all forest types, the authors report correlation values of r equals zero point eighty-three, zero point twenty-three, and zero point forty-two, respectively, with the organic-horizon relationship marked as statistically significant. This matters because it supports the organic horizon as an important factor in variation among bud concentrations, while also highlighting weaker or more variable relationships involving mineral soil.

The study considered mineral-horizon soil properties such as radiocesium fixation ability and potassium and ammonium concentrations as possible factors in Tag-137Cs variability. At the four sampling sites, the soils were brown forest soils with granitic rocks as the surface geology, so radiocesium fixation potentials may have been high and may have had little influence on differences in root uptake.

The results currently suggest that mineral-horizon soil properties were insufficiently variable to cause the large Tag-137Cs differences observed at the sampling sites. Figure five tracks the percentage of total cesium-one-thirty-seven deposition retained in the forest organic horizon over the years after the Fukushima Daiichi accident, separately for pine, cedar, and broad-leaved deciduous forests.

In all three panels, the dashed exponential curves and gray ninety-five-percent confidence intervals summarize a decline over time, while the points show observations from previous and present studies. This matters because organic-horizon contamination is linked to cesium activity in Koshiabura buds and may help reduce uncertainty in assessing their contamination.

The 137Cs activity concentration in Koshiabura buds had a significant positive correlation with 137Cs inventories only in the organic soil horizon. The organic-to-total 137Cs inventory ratio varied substantially, especially in broad-leaved deciduous forests where Koshiabura primarily lives.

That variation may explain the wide range of Tag-137Cs values calculated from total soil deposition. Understanding radioactive contamination in the organic horizon may reduce uncertainty in estimating Koshiabura-bud 137Cs activity concentration.

The study calculated a possible new aggregated transfer factor based on 137Cs inventory in the organic horizon, with a value of 0.075 plus or minus 0.037 square meters per kilogram. The bud concentration showed a strong significant positive correlation with organic-horizon 137Cs concentration, with r equal to 0.80 and p less than 0.001.

Figure two plots caesium one hundred thirty-seven activity in Koshiabura buds against soil deposition, with both publicly available data and measured point-scale data shown on logarithmic axes. The figure reports positive, statistically significant relationships for both groups: publicly available data have p less than zero point zero zero one and r equals zero point four one, while measured data have p equals zero point zero zero five and r equals zero point five eight.

The considerable scatter shows that soil deposition explains only part of the variation in bud activity. The strongest signal came from radiocesium in the organic soil horizon, not the deeper mineral horizon. For Koshiabura, matching the plant’s roots to the soil layer where cesium remains available may improve contamination estimates.