Effects of bark washing and epiphytic moss on 137Cs activity concentration in bark and stemflow in Japanese persimmon (Diospyros kaki Thunb.)

Abstract

The effects of bark washing, and of epiphytic moss growing on the bark surfaces, on the 137Cs activity concentrations in bark and stemflow samples were evaluated at two Japanese persimmon orchards located in Fukushima City and Date City, both approximately 60–65 km northwest of the Fukushima Dai-ichi nuclear power plant. The means of 137Cs in stemflow captured by 1 g of sphagnum placed on the main trunks of washed and unwashed trees, and on sites with moss and with no moss growing on bark of the non-washed trees in Date City, were 0.37 Bq, 1.08Bq, 0.77 Bq and 0.52 Bq respectively. The 137Cs was significantly lower in the washed bark and in the absence of moss at p = 0.00031 and p = 0.0443 by t-test respectively. The 137Cs in directly-collected stemflow on the main trunks was significantly higher for the unwashed tree at 6.0 Bq L−1 compared with the washed tree at 1.1 Bq L−1 (p = 0.017 by Tukey's test). The extraction rate of dissolved 137Cs with deionized water was 7.3% even after stirring for 40 h. These results indicate that the moss acts as reservoir of 137Cs, and that bark-washing decontamination can reduce the additional 137Cs supply from moss in subsequent years following the radioactive fallout.