Animals & nature · 2026
How did radioactive cesium in fallen leaves change when they were suspended above the ground versus placed on the forest floor?
Radioactive cesium decreased in suspended leaves but increased in leaves on the forest floor. In an approximately two-year experiment with fallen konara oak leaves, the decrease was likely due to rain and melting snow washing out water-soluble cesium. The increase came from soil particles entering the bags, mostly carrying cesium that was difficult to extract. The findings do not provide a simple way to predict contamination across the forest floor.
Details
Researchers put fallen konara oak leaves in bags, either suspended above the ground or placed on the forest floor, to study how cesium-137 from the Fukushima Dai-ichi accident moves as leaves decompose. They also tested how readily cesium could be washed out of the retrieved samples.
The contrasting results reflected different processes. Suspended bags lost mainly water-soluble cesium, likely through rain, snow and spring snowmelt. Bags on the forest floor gained cesium as soil particles entered them. Much of that added cesium was difficult to extract, even with a chemical extraction solution, rather than readily washed away.
Cesium was less mobile than potassium, an element used for comparison, whether or not soil particles entered the bags. The authors suggest that this low mobility helps explain why cesium remains in the forest floor's layer of decomposing organic matter. However, large differences in the cesium remaining across this and earlier experiments show how difficult it is to predict its concentration and distribution on the forest floor.
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