Turnover of 137Cs in 'soil-tree' system: An experience of measuring the isotope flows in a Siberian conifer forest.

Little attention has been paid to the uptake of 137Cs in natural forests under low levels of the isotope fallout when no immediate ecological danger presents. Here we present the extended assessments of the soil-to-plant and canopy-to-litter flows of 137Cs recently evaluated in a native Siberian forested area. The area undergoes a typical after-fire long term succession, with light-conifer upper story being followed by undergrowth of Siberian fir and other dark-conifer species. The one-year-old needles of Siberian fir were found to accumulate the largest concentration of the isotope, 4.10 Bq/kg oven-dry weight during the first growth season, as compared with older needles that accumulated 4.67 Bq/kg oven-dry weight in 2-3 years of growth. Based on these data an approach was developed that, hypothetically, can allow one to estimate the 137Cs activity concentration in soil solutions. Direct activity measurements in the soil solutions were not possible. The isotope activity in soil solutions was estimated to be 0.0061-0.0105 Bq/L. Based on the original data from the litter fall the annual flow of the isotope from the upper canopy to on-ground litter was found to be 0.42-0.84 Bq/m2. The amount of 137Cs that returns yearly back from canopy with falling litter was estimated to be 0.012-0.015% of the total soil isotope content. A combination of the estimations obtained in our study with the values of global 137Cs fallout allowed us to assess the ages (the time of formation) of horizons of the soils in the area.