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 137 Cs + and placed into close contact with an active macroscopic rhizosphere of ryegrass for 4 days. The 137 Cs rhizospheric mobilization was strongly correlated with the sodium tetraphenylboron-extractable 137 Cs ( r = 0.94), supporting that K depletion in the rhizosphere is a capital driving force in 137 Cs uptake. The 137 Cs 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 137 Cs + in the rhizosphere. They further support that many 137 Cs-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.