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Forest, Range & Wildland Soils

Reconciling Change in Oi-Horizon Carbon-14 with Mass Loss for an Oak Forest

^a Environmental Sciences Division, Oak Ridge National Lab., Oak Ridge, TN 37831-6422
^b Center for Accelerator Mass Spectrometry, Lawrence Livermore National Lab., Livermore, CA 94550
^c Univ. of California, Irvine, CA 92697

^* Corresponding author (hansonpj{at}ornl.gov)

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.

Abbreviations: AMS, accelerator mass spectrometer • EBIS, Enriched Background Isotope Study • ORR, Oak Ridge Reservation • SE, standard error • SOM, soil organic matter • TVA, Tennessee Valley Authority

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