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 O-2 during peat oxygenation events. We employed push-pull tests with dissolved O-2 as reactant to determine pseudo-first-order rate constants of O-2 consumption (k(obs)) in anoxic peat soil of an unperturbed Swedish ombrotrophic bog. Dissolved O-2 was rapidly consumed in anoxic peat with a mean k(obs) of 2.91 +/- 0.60 h(-1), corresponding to an O-2 half-life of similar to 14 min. POM dominated O-2 consumption, as evidenced from approximately SO-fold smaller k obs in POM-free control tests. Inhibiting microbial activity with formaldehyde did not appreciably slow O-2 consumption, supporting abiotic O-2 reduction by POM moieties, not aerobic respiration, as the primary route of O-2 consumption. Peat preoxygenation with dissolved O-2 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 mu mol electrons donated per gram POM. This work demonstrates rapid abiotic oxidation of reduced POM by O-2, supporting that short-term oxygenation events can restore the capacity of POM to accept electrons from anaerobic respiration in temporarily anoxic parts of peatlands.