Isotopic ratio of 137Cs/133Cs in soils and vegetation cover of forest ecosystems (review)

Abstract

The aim of the work is to summarize the data available in the scientific literature on the values of the ¹³⁷Cs/¹³³Cs isotope ratio in soils and vegetation cover of forest ecosystems in radioactively contaminated and background territories. The values of the specific activity of ¹³⁷Cs and the concentration of ¹³³Cs presented in radioecological studies are unified by calculating the molar ratio of these isotopes. Based on the generalized literature data, it is shown that the values of ¹³⁷Cs/¹³³Cs in soils, forest litter and vegetation increase in radioactively contaminated areas. In background regions exposed only to global radioactive fallout, the isotopic ratio of ¹³⁷Cs/¹³³Cs is about 0,1–1,0 (× 10^-8), whereas in the Chernobyl fallout zone it increases by 1000–100 000 times. The isotopic ratio of ¹³⁷Cs/¹³³Cs decreases down the soil profiles in various forest ecosystems. Spatial variation of the ¹³⁷Cs/¹³³Cs ratio in litter is associated with the diversity of ecotopes in forest biogeocenoses. The alignment of the values of the ¹³⁷Cs/¹³³Cs ratio in the vegetation cover and upper soil horizons indicates the achieved equilibrium of technogenic ¹³⁷Cs with natural stable ¹³³Cs in the biogeochemical cycle of this element. A comparative study revealed that the isotopic ratio of ¹³⁷Cs/¹³³Cs increases in the following range of plant cover representatives: Bryophyta < Sorbus aucuparia < Lichenes < Russula vesca < Frangula alnus < Polypodiophyta. For fungi, interspecific differences were noted in the following increasing series in the isotopic ratio ¹³⁷Cs/¹³³Cs: Lactarius deliciosus < Sarcodon scabrosus < Suillus bovinus < Tricholoma saponaceum < Lepista nuda < Russula delica < Macrolepiota excoriata. For the reference species of coniferous trees — Pinus sylvestris - the isotopic ratio of ¹³⁷Cs/¹³³Cs in the forest area of the Bryansk region polluted by Chernobyl fallout increases in the following number of components: roots < large branches < trunk wood < needles < small branches < cones.