Efficient sampling of shiitake-inoculated oak logs to determine the log-to-mushroom transfer factor of stable cesium

Stable cesium (¹³³Cs) naturally exists in the environment whereas recently deposited radionuclides (e.g., ¹³⁷Cs) are not at equilibrium. Stable cesium has been used to understand the long-term behavior of radionuclides in plants, trees and mushrooms. We are interested in using ¹³³Cs to predict the future transfer factor (TF) of radiocesium from contaminated logs to shiitake mushrooms in Eastern Japan. However, the current methodology to obtain a representative wood sample for ¹³³Cs analysis involves mechanically breaking and milling the entire log (excluding bark) to a powder prior to analysis. In the current study, we investigated if sawdust obtained from cutting a log along its length at eight points is as robust but a faster alternative to provide a representative wood sample to determine the TF of ¹³³Cs between logs and shiitake.

Oak logs with ready-to-harvest shiitake fruiting bodies were cut into nine 10-cm discs and each disc was separated into bark, sapwood and heartwood and the concentration of ¹³³Cs was measured in the bark, sapwood, heartwood, sawdust (generated from cutting each disc) and fruiting bodies (collected separately from each disc), and the wood-to-shiitake TF was calculated.

We found that the sawdust-to-shiitake TF of ¹³³Cs did not differ (P = 0.223) compared to either the sapwood-to-shiitake TF or heartwood-to-shiitake TF, but bark did have a higher concentration of ¹³³Cs (P < 0.05) compared to sapwood and heartwood. Stable cesium concentration in sawdust and fruiting bodies collected along the length of the logs did not differ (P > 0.05).

Sawdust can be used as an alternative to determine the log-to-shiitake TF of ¹³³Cs. To satisfy the goals of different studies and professionals, we have described two sampling methodologies (Methods I and II) in this paper. In Method I, a composite of eight sawdust samples collected from a log can be used to provide a representative whole-log sample (i.e., wood and bark), whereas Method II allows for the simultaneous sampling of two sets of sawdust samples-one set representing the whole log and the other representing wood only. Both methodologies can greatly reduce the time required for sample collection and preparation.