First‐year litter decomposition was estimated for an upland‐oak (Quercus spp.) forest ecosystem using enrichment or dilution of the 14C‐signature of the Oi‐horizon. These isotopically based mass‐loss estimates were contrasted with measured mass‐loss rates from past litterbag studies. Mass‐loss derived from changes in the 14C‐signature of the Oi‐horizon suggested mean mass loss over 9 mo of 45%, which was higher than the corresponding 9‐mo rate extrapolated from litterbag studies (∼35%). Greater mass loss was expected from the isotopic approach because litterbags are known to limit mass loss processes driven by soil macrofauna (e.g., fragmentation and comminution). Although the 14C‐isotope approach offers the advantage of being a non‐invasive method, it exhibited high variability that undermined its utility as an alternative to routine litterbag mass loss methods. However, the 14C approach measures the residence time of C in the leaf litter, rather than the time it takes for leaves to disappear; hence radiocarbon measures reflect C immobilization and recycling in the microbial pool, and do not necessarily replicate results from litterbag mass loss. The commonly applied two‐compartment isotopic mixing model was appropriate for estimating decomposition from isotopic enrichment of near‐background soils, but it produced divergent results for isotopic dilution of a multi‐layered system with litter cohorts having independent 14C‐signatures. This discrepancy suggests that cohort‐based models are needed to adequately capture the complex processes involved in C transport associated with litter mass‐loss. Such models will be crucial for predicting intra‐ and interannual differences in organic horizon decomposition driven by scenarios of climatic change.