Effect of Potassium Application on Root Uptake of Radiocesium in Rice
To decrease radiocesium uptake in brown rice from the contaminated fields, we examined the effect of using potassium fertilizer on the radiocesium uptake in brown rice.
Rice plants (Oryza sativa) were cultivated in the five experimental paddy fields in the northern area in Fukushima Prefecture, which was contaminated with radiocesium, in 2011. Soils and rice plants were collected from five points in each experimental field at harvest time. The mean concentration of radiocesium at soil depths of 0–5, 5–10, and 10–15 cm was 5,800, 3,200 and 1,800 Bq kg−1 dry weight (DW), which was not uniformly distributed, even during plowing before cultivation. The concentration of radiocesium in each field at the depth of 0–5 cm showed approximately threefold variation.
The concentration of radiocesium in brown rice from five rice paddy fields was 231 ± 135 (52–485) Bq kg−1 fresh weight (FW), and the values were different in each collecting point. The soil-to-plant transfer factor is a simple but important parameter that can be used to estimate the concentrations of radionuclides in plants. The transfer factor generally shows a very wide range of variation. The transfer factor of soilto-brown rice collected from a pair of soil and brown rice samples at each point was in the range of 0.0075–0.11, which was more than one order of magnitude different. Tsukada et al. and Komamura and Tsumura reported that the geometric mean of the soil-to-plant transfer factor of polished rice in rice paddy fields, which were determined by the fallout depositions derived from the nuclear weapons tests, as 0.0016 and 0.0030, respectively [3, 4]. The observed values, which were determined in the same or nearby fields, were higher than previously reported values. The difference may be attributed to nonuniform distribution of radiocesium in the available fractions because of the early stage of the aging periods after deposition onto the soil.
Potassium is an important essential element in plant physiology, and it is supplemented by the application of fertilizers to agricultural soils. There was a high correlation (r = 0.88) between the soil-to-brown-rice transfer factor of radiocesium and the exchangeable K2O in the soil (Fig. 13.1). Other researchers have reported that the transfer factor of 137Cs decreased with increasing concentrations of potassium in soils [5–7]. Kato  also reported that the soil-to-plant transfer factor of radiocesium decreased with increasing concentrations of exchangeable K2O in soils. Further, the soil-to-brown-rice transfer factor of radiocesium also decreased from 0.074 to 0.024 with the application of potassium fertilizer through top dressing. Hence, it is clear that the application of potassium fertilizer reduces the concentration of radiocesium in brown rice.