Fractionation of U, Th, Ra and Pb from boreal forest soils by sequential extractions

•A five-step sequential extraction was performed to boreal forest soil samples.•Extractions gave information about mobility and sorption of natural radionuclides.•Fractionation of U, Th, Pb and Ra varied between the sample depth and grain size.•U, Pb and Fe fractionation suggested more reducing conditions in the subsoil samples.•Ra was the only radionuclide that could be regarded as mobile.