Spatial distribution of ¹³⁷ Cs concentrations in mushrooms (Boletus hiratsukae) and their relationship with soil exchangeable cation contents

The activity concentrations of radiocesium (¹³⁷Cs) in wild mushrooms are reported to vary according to species, genus or ecological types. In addition, the concentration fluctuates among the same species collected within the same area. Therefore, we investigated whether the ¹³⁷Cs concentration of wild mushrooms would be (1) spatially biased, or (2) influenced by the ¹³⁷Cs or exchangeable potassium concentrations in the soils below. We set two survey plots 300 m apart in a Himalayan cedar forest in Tsukuba, Japan, where ca. 30 kBq/m² of ¹³⁷Cs was deposited after the Fukushima Nuclear Power Plant accident. From these plots, we collected fruit-bodies (fungal structures for spore production) of co-occurring Boletus hiratsukae, a mycorrhizal species, as well as from the soil below. The mean ¹³⁷Cs concentrations in the fruit-bodies were significantly different between the two plots, but no difference was observed in the soil ¹³⁷Cs concentration between the plots. Significant spatial autocorrelation was observed in the ¹³⁷Cs concentration in the organic layer for both sites, but no significant spatial autocorrelation was observed in the ¹³⁷Cs of fruit-bodies. Therefore, the variation in the ¹³⁷Cs concentrations of co-occurred B. hiratsukae was not explained by spatial bias or radioactivity in the below soil. In contrast, the exchangeable potassium concentration in the soil was negatively correlated with the ¹³⁷Cs in the fruit-bodies. Our results suggest that the ¹³⁷Cs absorption of wild mushrooms would be suppressed by the competitive effect of exchangeable potassium in the surrounding soils.

Keywords: Boletus hiratsukae; Exchangeable potassium; Mycorrhizal fungi; Radiocesium; Spatial autocorrelation.