Igor Andreevich Grabeklis

In the paper, the features of the biological cycle of 90Sr in the birch forests of the Bryansk region in the remote period
aft er the Chernobyl fallout are discussed. Specific activity and inventories of 90Sr in soil, diff erent structures and types
of tree and herbaceous-shrubby stories, higher fungi, and litter are given. It is shown that at present the major part
of 90Sr in the contaminated birch forests of the Bryansk Polesie is accumulated in the soil (82,3%), of which about
71% is in the mineral soil horizons. The biota contains only 17,7% of the total radionuclide inventory with maximum
values in the tree story. Th e contribution of herbaceous-shrubby vegetation and higher fungi is small, <0,5%. In birch
forests 3,07 kBq·m-2 90Sr is involved in the biological cycle, which is about 3% of its total stock in the ecosystem and
approximately corresponds to the radioactive decay of this radionuclide. The main part of the 90Sr goes for formation
of annual growth of the stand, while the contribution of higher fungi to the cycle exceeds the contribution of the
herbaceous-shrubby layer. Th e 90Sr return to soil is lower (48,2%) than its fi xation in annual biomass growth (51,8%).