A forest can look less contaminated on paper, yet its edible shoots can still carry unexpectedly high radioactivity. The reason may be not how much is in the soil, but where in the soil it sits. After a nuclear accident, radioactive cesium can keep circulating through a forest and contaminate mushrooms, berries, game animals, and other forest products for a long time.

Many areas affected by the Fukushima accident are forested, and most have not yet been cleaned up. Prolonged contamination remains a major concern for the region’s social and economic recovery. The study focused on Koshiabura buds at the edible stage and asked what factors explain variation in radioactive cesium between the buds and the soil.

The key questions were whether the answer changes with the size of the area used to describe soil contamination, and whether cesium deeper in the soil matters as much as cesium near the surface. With airborne survey data, using a more spatially limited administrative-district average rather than a municipal average tended to reduce variation in the calculated cesium values.

But measuring soil directly near the plants did not reduce the variation compared with using the more limited area average. That was the first surprise: even a more accurate picture of total soil contamination did not remove the differences in radioactive cesium found in the buds.

The overall contamination of the soil is therefore not always directly reflected in the radioactive cesium concentration of Koshiabura buds. The stronger clue came from soil depth. Cesium in the organic horizon, the thin surface layer made from decaying plant material, had a significant positive relationship with the amount in the buds.

That relationship was stronger than the one involving the mineral soil down to ten centimetres, even though the mineral layer held much more cesium. The likely reason is that cesium in organic matter is more available to plants, while Koshiabura’s fine roots are mostly in the surface layer.

The bud contamination tracks the radioactive material held in the surface organic layer, but not the amount buried deeper in mineral soil. That points to shallow roots as an important reason contamination varies from place to place.

The final piece is that the share of all soil cesium held in the organic surface layer varied greatly, especially in the broad-leaved forests where Koshiabura mainly grows. Because the organic layer's share of total soil cesium varied markedly, estimates of cesium in Koshiabura buds based on total soil deposition varied widely too.

Measuring contamination in that organic surface layer may reduce uncertainty when estimating radioactive cesium in Koshiabura buds. The study found a strong positive relationship between cesium in the buds and cesium in the organic horizon, suggesting that this relationship can help determine the amount in the buds.

Because cesium varies among wild edible plants, the study recommends assessing transfer indicators using the plant's root-system distribution alongside cesium's vertical distribution in the soil. For Koshiabura, the radioactive material in the thin organic layer at the soil surface tracks the buds better than the total amount in the ground.

That could make safety estimates more useful for people who gather or buy forest foods.