Reoxidation of Reduced Peat Organic Matter by Dissolved Oxygen: Combined Laboratory Column‐Breakthrough Experiments and In‐Field Push‐Pull Tests

Electron transfer to peat particulate organic matter (POM) as terminal electron acceptor (TEA) in anaerobic respiration has been hypothesized to lower methane emissions from peatlands by competitively suppressing methanogenesis and/or allowing for anaerobic oxidation of methane. We herein provide evidence for two critical aspects of this hypothesis: (a) peat POM is present in a reduced state in situ in the anoxic peat subsurface, and (b) reduced POM at the oxic-anoxic interface in peat soils can be oxidized by dissolved oxygen (DO), restoring its TEA capacity. We reacted reduced POM from three ombrotrophic bogs in Sweden with DO in soil-packed column-breakthrough experiments (CBEs), mimicking oxidation reactions at the oxic-anoxic interface. Breakthrough of DO was substantially retarded relative to the inert tracer bromide, consistent with DO reduction by POM. Control experiments confirmed abiotic DO reduction and excluded DO consumption through aerobic respiration. Modeling DO breakthrough revealed fast and slowly reacting POM moieties with reaction times spanning minutes to days. Complementary push-pull tests (PPTs) in the anoxic subsurface of one bog confirmed results from laboratory CBEs: lower recoveries of injected DO compared with bromide supported DO reduction by reduced POM. Rates of reduction of injected DO decreased with increasing number of injections, consistent with continuous oxidation of reduced POM. Electron-donating capacities of POM to DO in CBEs and PPTs were comparable at 40–130 μmol e−/g dry POM. Our results substantiate that POM is a regenerable TEA at oxic-anoxic interfaces in peat soils and, thereby, may substantially lower CH4 emissions from peatlands