Oxidation of Reduced Peat Particulate Organic Matter by Dissolved Oxygen: Quantification of Apparent Rate Constants...

Peat particulate organic matter (POM) is an important terminal electron acceptor for anaerobic respiration in northern peatlands provided that the electron-accepting capacity of POM is periodically restored by oxidation with O₂ during peat oxygenation events. We employed push-pull tests with dissolved O₂ as reactant to determine pseudo-first-order rate constants of O₂ consumption ( k_obs) in anoxic peat soil of an unperturbed Swedish ombrotrophic bog. Dissolved O₂ was rapidly consumed in anoxic peat with a mean k_obs of 2.91 ± 0.60 h^-1, corresponding to an O₂ half-life of ∼14 min. POM dominated O₂ consumption, as evidenced from approximately 50-fold smaller k_obs in POM-free control tests. Inhibiting microbial activity with formaldehyde did not appreciably slow O₂ consumption, supporting abiotic O₂ reduction by POM moieties, not aerobic respiration, as the primary route of O₂ consumption. Peat preoxygenation with dissolved O₂ lowered k_obs in subsequent oxygen consumption tests, consistent with depletion of reduced moieties in POM. Finally, repeated oxygen consumption tests demonstrated that anoxic peat POM has a high reduction capacity, in excess to 20 μmol electrons donated per gram POM. This work demonstrates rapid abiotic oxidation of reduced POM by O₂, supporting that short-term oxygenation events can restore the capacity of POM to accept electrons from anaerobic respiration in temporarily anoxic parts of peatlands.