Oxidation of Reduced Peat Particulate Organic Matter by Dissolved Oxygen - Combining Laboratory with In Situ Studies

Ombrotrophic bogs are anoxic, carbon-rich ecosystems that are rainwater-fed and hence deprived of nutrients. Consequently, inorganic terminal electron acceptors (TEAs), such as nitrate and sulfate, are scarce and methanogenesis is expected to be the final step in the degradation of organic matter. However, numerous studies have reported molar CO2:CH4 production ratios much larger than unity, indicating that anaerobic respiration prevails over methanogenesis. It has been hypothesized that respiration utilizes oxidized peat particulate organic matter (POM) as previously unrecognized TEA, resulting in POM reduction. The oxidized POM is expected to be regenerable through reoxidation of reduced POM by O2 entering the peat soil, for instance during water-table drawdowns. Here, we present results from combined laboratory and in situ studies of electron transfer from POM to dissolved O2 in peat soils to demonstrate that POM is indeed present in reduced state in the anoxic subsurface, and that POM can be reoxidized by O2. In situ studies consisted of push-pull tests conducted over the course of two weeks, in which we daily injected O2-saturated solutions into the anoxic subsurface and, following a specific reaction time, retrieved parcels to determine the abiotic consumption of O2. Our results show that POM is present in reduced state in situ and can transfer about 49 to 76 mmol e- per g POM to dissolved O2. However, oxidations over two weeks were insufficient to result in complete reoxidation of POM. In complementary laboratory column breakthrough experiments, we studied the oxidation of reduced POM (packed into the columns in the field) by pumping O2-saturated solutions through the columns and determining decreases in O2 concentrations from the in- to the outflow of the column. We show that POM can transfer about 71 to 101 mmol e- per g POM. Reoxidation kinetics by O2 are well-described by modeling the data with two pools of reduced moieties in the POM: fast- and slow reacting POM pool. Altogether, our results provide the first quantitative measure of the number of electrons that can be abiotically transferred from reduced POM to dissolved O2 and provide insight into the kinetics of this process, both in situ and in the laboratory. Our results fully support considering POM as a TEA to understand carbon dynamics in ombrotrophic bogs.