Distribution of radioactive cesium in soil and its uptake by herbaceous plants in temperate pastures with different management after the Fukushima Dai-Ichi Nuclear Power Station accident

The accident at Fukushima Dai-Ichi Nuclear Power Station (NPS) extensively contaminated the agricultural land in the Tohoku region of Japan with radioactive cesium [sum of cesium-134 (¹³⁴Cs) and cesium-137 (¹³⁷Cs)]. We evaluated the status of radioactive cesium (Cs) contamination in soil and plants at the Field Science Center of Tohoku University, northern Miyagi prefecture, 150 km north of the NPS. In seven pastures with different management, we examined: (1) the distribution of radioactive Cs in soil, (2) the concentration of radioactive Cs in various herbaceous plant species and (3) the change in radioactive Cs content of plants as they matured. We collected samples of litter, root mat layer (root mat soil and plant roots), and subsurface soil (0–5 cm beneath the root mat) at two to three locations in each pasture in December 2011 and May 2012. The aboveground parts of herbaceous plants (four grasses, two legumes, and one forb species) were collected from May 9 to June 20, 2012, at 14-d intervals, from one to five fixed sampling locations in each pasture. The distribution of radioactive Cs in soil differed among pastures to some degree: a large proportion of radioactive Cs was distributed in the root mat layer. Pasture management greatly influenced the radioactive Cs content of herbaceous plants (p < 0.001); plant species had less influence. Radioactive Cs content was highest (> 3 kBq kg⁻¹dry weight) on May 9 and significantly decreased with maturity (p < 0.001) for most of the pastures, whereas it remained low (0.04–0.18 kBq kg⁻¹dry weight) throughout the measurement period in the pasture where composted cattle manure was applied. The soil-to-plant transfer factor was negatively correlated to pH(H₂O) (R² = 0.783, p < 0.001) and exchangeable K content (R² = 0.971, p < 0.001) of root mat soils, which suggests that surface application of composted cattle manure reduces plant uptake of radioactive Cs by increasing the exchangeable K content of the soil. The radioactive Cs content of plants decreased with plant maturity; its degree of decrease (May 9 to June 6) was smaller in legumes (80.6%) than grasses (55.5%) and the forb (58.6%). Radioactive Cs content decreased with plant maturity; also, the proportion remaining in the aboveground plant was higher in legumes (80.6%) than grasses (55.5%) and the forb (58.6%).