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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 137 Cs and 90 Sr. Transfer of 137 Cs from plant roots and concentration on the surface of soil aggregates have been established experimentally. Indirect data also point to the potential localization of 137 Sc 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. 137 Cs 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 90 Sr, 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, 90 Sr 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 137 Cs by one to two orders of magnitude) favors the permanent renewal of loci with increased 90 Sr concentrations in the inter-and intraaggregate pore space in the form of plant residues.

In model experiments under forest ecosystem conditions, samples of partially decomposed forest litter and soil from a 0–10 cm layer containing 14C-labeled organic matter were selected. The label was introduced in the form of low molecular weight water-soluble organic substances of individual nature – glycine, uracil and glucose together with unlabelled plant litter in nylon bags. The experiment was carried out in the late autumn period on the territory of the Malinsky forestry, 20 km south-west of Moscow, on a medium loamy sod-podzolic soil. For the study, samples were taken from two selection periods – 40 days after labelling and after two years. The selected samples were extracted and separated into soil organic matter fractions containing 14C in their composition, followed by radiometry. The results showed differences in the transformation and incorporation of radiocarbon of the studied products – glycine, uracil, and glucose into different fractions of soil organic matter of low molecular weight water-soluble substances, depending on their nature.

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