This study investigated the log-to-shiitake transfer factor (TF) for stable cesium (133Cs) in a controlled cultivation setting. Stable cesium (133Cs) is used as an analog for the long-term behavior of radiocesium because it is naturally present in the environment and is in equilibrium, whereas recently deposited radiocesium (such as 137Cs) is not. The practical context is quantifying how much cesium moves from contaminated logs to shiitake fruiting bodies in Eastern Japan.
Key numbers
133Cs concentrations in log components (Table 1, p. 7-8):
- Heartwood (5 logs, n = 9 discs/log): mean 27 ± 19.3 µg/kg dry weight (DW); range 13-122 µg/kg DW.
- Sapwood (5 logs): mean 28 ± 19.5 µg/kg DW; range 10-127 µg/kg DW.
- Bark (5 logs): mean 71 ± 32.7 µg/kg DW; range 34-251 µg/kg DW; significantly higher than heartwood or sapwood (P < 0.05, Tukey post-hoc test).
- Sawdust (10 logs, n = 8 cuts/log): mean 26 ± 14.1 µg/kg DW; range 6-93 µg/kg DW.
133Cs concentrations in shiitake fruiting bodies (Table 1, p. 7-8):
- Logs 1-5: mean 677 ± 358.4 µg/kg DW; range per log 383-1255 µg/kg DW.
- Logs 1-10: mean 591 ± 319.3 µg/kg DW; range per log 258-1255 µg/kg DW.
- Concentration in fruiting bodies from odd-numbered disc positions (600 µg/kg DW) did not differ significantly from even-numbered positions (581 µg/kg DW; P > 0.05, paired t-test) (Figure 3C, p. 10).
Log-to-shiitake transfer factors for 133Cs (DW basis, Table 1, p. 7-8):
- Sawdust-to-shiitake TF (10 logs): mean 23 ± 4.0.
- Sawdust-to-shiitake TF (5 logs): mean 24 ± 3.3.
- Heartwood-to-shiitake TF (5 logs): mean 29 ± 6.9.
- Sapwood-to-shiitake TF (5 logs): mean 27 ± 3.3.
- TF on fresh weight basis: 4.2 (sawdust to shiitake).
- Sawdust-to-shiitake TF did not differ significantly from heartwood-to-shiitake or sapwood-to-shiitake TF (P = 0.223).
For comparison, the radiocesium (137Cs) log-to-shiitake TF was reported in prior literature as 0.43 (from semi-dried logs, fresh weight basis), substantially lower than the 133Cs TF of 23 on a DW basis. The authors attribute the difference to the fact that 137Cs was distributed primarily in bark (the most recently contaminated compartment) at the time of post-accident measurement, whereas 133Cs is distributed throughout the log.
Methods (brief)
Ten Quercus serrata oak logs (mean weight 5 ± 1.0 kg; mean length 94 ± 0.9 cm; mean diameter 9 ± 1.0 cm) were sourced from Sano-city, Tochigi Prefecture, inoculated with Lentinula edodes strain F103 and fruited in January-February 2017; first-crop fruiting bodies ready for harvest were collected in March 2018. Each log was cut at nine 10-cm intervals; sawdust from each cut was collected. For five logs, discs were further separated into bark, sapwood, and heartwood. All samples were dried, crushed, milled, and analyzed for 133Cs by ICP-MS (PerkinElmer NexION 350S) using cesium standard solution (998 mg/L) and indium internal standard.
Evidence fitness
This paper provides stable cesium concentrations in shiitake fruiting bodies and in the oak log substrate under controlled cultivation conditions in Tochigi Prefecture. The transfer factors characterize the efficiency of cesium movement from wood to mushroom, relevant to understanding why shiitake from contaminated logs accumulate cesium. The paper does not report radioactive cesium (137Cs or 134Cs) concentrations, does not examine consumer-level dietary intake, and was conducted in a region outside the Fukushima difficult-to-return zone.
Limitations
- Sample origin (Sano-city, Tochigi) is outside the highest-contamination zones; stable cesium concentrations in logs reflect ambient geochemical levels, not post-accident contamination levels.
- Only first-crop fruiting bodies were measured; cesium concentrations may differ across later crops.
- All logs from a single grower; site-specific variation in stable cesium soil concentrations is not captured.
- No data on how 133Cs concentrations in logs or shiitake compare to regions with higher or lower stable cesium soil levels.
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