Respective horizon contributions to cesium-137 soil-to-plant transfer: A rhizospheric experimental approach

In forest soils polluted by radiocesium, the surface horizons are known to contribute differently to Cs plant contamination. A precise quantification of the horizon contributions is, however, seldom realized. We quantified the respective contributions of the Of, OAh, Ah, and Bw horizons in an acid brown soil to the total ¹³⁷Cs soil-to-plant transfer through a rhizospheric approach. The macroscopic rhizosphere was realized by creating a close contact between ¹³⁷Cs- contaminated soil horizons earlier mixed with agar and a dense root mat previously developed by young, K-stressed ryegrass seedlings (Lolium multiflorum Lam., cv Lemtal) during a period of 7 d. The intimate root-soil contact was maintained for 4 d. The uptake of radiocesium by plant roots was readily effective as the ¹³⁷Cs rhizospheric mobilization (RM) amounted to 0.07 to 23.42% of the initial ¹³⁷Cs soil contamination. Assuming negligible horizon to horizon transfer and equivalent root exploration in each horizon, the respective contributions of the horizons to the ¹³⁷Cs soil-to-plant transfer were 96.7% in Of, 0.13% in OAh, 1.34% in Ah, and 1.84% in Bw. Our data confirm the very high contribution of the organic horizon in the ¹³⁷Cs soil-to-plant transfer. They further suggest that the rhizospheric approach could be well suited to classify soils for their potential effect on ¹³⁷Cs plant contamination.