Reconciling change in Oi-horizon carbon-14 with mass loss for an oak forest

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.