Content and Behavior of Heavy Natural Radionuclides in Tula Zaseki Gray and Agro-Gray Soils

The 226 Ra, 238 U, and 232 Th fractions have been studied in the virgin gray soil under the forest and in the agro-gray soil of the Tula forest entanglements (Tula Zaseki). Radionuclides have been fractionated by Pavlotskaya’s sequential extraction method. The 238 U and 232 Th concentrations have been determined with an inductively coupled plasma quadrupole mass spectrometer. The 226 Ra activity has been measured using an alpha-radiometer, taking into account the decay of radium isotopes and the accumulation of their daughter decay-related products. Long-term (decades) and systematic use of phosphorus fertilizers leads to an increase in 226 Ra, 238 U, and 232 Th reserves in the ploughed layer (0–25 cm) of the agro-gray soil to levels of 15.1, 15.4, and 16.2 kBq/m 2 , respectively, which is 1.7, 1.4, and 1.1 times higher than in the virgin gray soil under the forest. Alongside with that, a considerable amount of radionuclides (21.9% 226 Ra, 9.8% 238 U, and 4.8 % 232 Th entering the soil along with phosphate fertilizers) remains available to plants due to their accumulation in the exchangeable fraction. Among the potentially mobile fractions for 238 U and 232 Th forms, along with the exchangeable fraction, the mobile fraction “proper” (extracted by 1M HCl) is also important.