Rhizospheric mobilization of radiocesium in soils

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