Oxidation of Reduced Peat Particulate Organic Matter by Dissolved Oxygen: Quantification of Apparent Rate Constants in the Field

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.