For better understanding of soil-to-plant transfer of radionuclides, their behavior in a soil solution should be elucidated, especially at the interface between plant roots and soil particles, where conditions differ greatly from the bulk soil because of plant activity (nutrient uptake and root exudations of H⁺, OH^- and organic compounds, etc.). This study examined effects of plant growth on concentration of stable Cs and Sr, and U in soil solutions.

Methods

Volcanic ash soil (pH(H₂O) 5.5; total carbon contents, 85 g kg^-1; Cs, 3.3 mg kg^-1; Sr, 121 mg kg^-1; U, 2.4 mg kg^-1) collected from an arable field was used for a pot cultivation experiment. Soil samples (400 g dry soil) were put into plastic pots after mixing with NH₄NO₃ and KH₂PO₄ fertilizer reagents (N, 60 mg/pot; P, 17 mg/pot). In the pots, the rhizosphere zone (R) was separated from the non-rhizosphere zone (NR) by a nylon net screen (20 μm). Komatsuna (Brassica rapa L.) was cultivated in an environmentally controlled growth chamber (temperature, 20°C; relative humidity, 70%; day length, 12 h; light intensity, 20,000 lx) for 26 days. The water content in soil was adjusted to 70% of its maximum water-holding capacity by adding ultrapure water every day. Soil solutions in the planted pot (R and NR) and in the unplanted pot (UP) were collected at 5-day intervals by high-speed centrifugation method, and filtered through a membrane filter (0.4 μm). The Sr concentration was measured using ICP-AES, and those of Cs and U were determined using ICP-MS.

Results

The Cs concentration in the UP soil solution decreased with time (0.73–0.05 μg L^-1), and was correlated with the NH₄ concentration in the soil solution. Results suggested that the Cs released from the soil particles by exchange with NH₄ added as fertilizer, then was re-absorbed onto the soil particles with time elapsing. The Cs concentration in the soil solution at NR zone did not differ from that at the UP. The Cs concentration at the R zone was lower than that at NR and UP through the cultivation period (0.27–0.01 μg L^-1). For that reason, the plant should uptake Cs only from the soil solution that is close to the root. The Sr concentration in the soil solution at the R zone also decreased with plant growth (140–17 μg L^-1). The Sr concentration in the soil solution at the NR zone was lower than that at UP in the later growth stage. The Sr in the soil solution that was distant from plant root was able to move to the root zone along with the water mass flow. On the other hand, the concentration of U in the soil solution at R zone increased with plant growth (0.03–0.16 μg L^-1). The dissolved organic carbon (DOC) concentration in the soil solution at the R zone also increased with plant growth during the cultivation period (44–80 mg kg^-1). Therefore, organic substances increasing with plant growth should play an important role in U behavior in the rhizosphere.

This study was supported by a grant from the Aomori Prefectural Government.