The importance of root systems for C cycling depends crucially on fine root longevity. We investigated mean values for fine root longevity with root diameter, root C/N ratio and soil depth using radiocarbon (¹⁴C) analyses in a temperate Norway spruce [Picea abies (L.) Karst.] forest. In addition, we applied sequential soil coring and minirhizotron observations to estimate fine root longevity in the organic layer of the same stand. The mean radiocarbon age of C in fine roots increased with depth from 5 years in the organic layer to 13 years in 40–60 cm mineral soil depth. Similarly, the C/N ratios of fine root samples were lowest in the organic layer with a mean value of 24 and increased with soil depth. Roots >0.5 mm in diameter tended to live longer than those being <0.5 mm in diameter. By far the strongest variability in fine root longevity estimates was due to the chosen method of investigation, with radiocarbon analyses yielding much higher estimates (5.4 years) than sequential soil coring (0.9 years) and minirhizotron observations (0.7 years). We conclude that sequential soil coring and minirhizotron observations are likely to underestimate mean fine root longevity, and radiocarbon analyses may lead to an overestimation of mean root longevity.