Rhizospheric Mobilization of Radiocesium in Soils

Abstract

Though soil−plant transfer is the first step by which radiocesium enters the food chain, it has been scarcely studied in the rhizosphere. Forty-seven soil horizons from 17 pedons with widely varying properties were contaminated with carrier-free 137Cs+ and placed into close contact with an active macroscopic rhizosphere of ryegrass for 4 days. The 137Cs rhizospheric mobilization was strongly correlated with the sodium tetraphenylboron-extractable 137Cs (r = 0.94), supporting that K depletion in the rhizosphere is a capital driving force in 137Cs uptake. The 137Cs soil−plant transfer factor varied from 0.02 to 3.69 g g-1 between soil materials and was strongly negatively correlated to the radiocesium interception potential (RIP) (r = −0.88), a common Cs binding characteristic in soil. RIP largely differed between soil materials (13−4861 μmol g-1) and was directly related with the soil vermiculite content (r = 0.70). Our results, validated in a wide variety of soils, show that both vermiculitic minerals and plant roots act as competitive sinks for 137Cs+ in the rhizosphere. They further support that many 137Cs-polluted soils in semi-natural environments can act as a potential source for long-term contamination of the above standing vegetation because they have low K availability.