Role of plants in the spatial differentiation of 137Cs and 90Sr statuses on the aggregate level

Abstract

The increased concentration of an element in plant biomass compared to the soil mass is an essential condition for the differentiated spatial distribution and status of the element on the aggregate level. Two forms of this differentiation have been revealed for 137Cs and 90Sr. Transfer of 137Cs from plant roots and concentration on the surface of soil aggregates have been established experimentally. Indirect data also point to the potential localization of 137Cs on the surface of intraaggregate pores. The effect of radionuclide concentrating on the outer and inner surfaces of aggregates is due to the rapid and strong fixation of cesium microamounts by mineral soil components. 137Cs from the surface of aggregates is more available for the repeated uptake by plant roots than from the intraped mass. The distortion of this spatial differentiation mainly occurs during the reaggregation of soil mass, which in turn decreases the availability of the radionuclide to plants. For 90Sr, its elevated concentration in the form of organic residues has been revealed in the inter- and intraaggregate pore space. However, due to the high diffusion rate, 90Sr is relatively rapidly (during several months under pot experimental conditions) redistributed throughout the entire volume of soil aggregates and its major part gradually passes into the phase of humic compounds, to which the radionuclide is bound by exchange sorption. The high level of the next root uptake (higher than for 137Cs by one to two orders of magnitude) favors the permanent renewal of loci with increased 90Sr concentrations in the inter- and intraaggregate pore space in the form of plant residues.