Electron transfer to peat particulate organic matter in ombrotrophic bogs and implications for methane formation: a combined field and laboratory study

Ombrotrophic bogs are rainwater-fed, water-logged, anoxic, and carbon-rich ecosystems with low concentrations of dissolved inorganic terminal electron acceptors (TEAs), such as nitrate and sulfate. Consequently, methanogenesis is expected to dominate carbon turnover in many of these systems and to result in an approximatively equimolar formation of CO₂ and CH₄. Yet, numerous studies have reported elevated molar CO₂:CH₄ formation ratios in peat bog soil incubations, indicating that anaerobic respiration prevails over methanogenesis despite the apparent scarcity of inorganic TEAs. To explain anaerobic respiration, particulate organic matter (POM) was proposed to act as previously unrecognized TEA. Here, we present results from combined in situ field studies and laboratory peat soil incubations to assess electron transfer to oxidized POM (POM_ox) and its effects on CO₂ and CH₄ formation. In situ studies consisted of deploying litter mesh bags containing POM_ox in the anoxic, water saturated subsurface of three ombrotrophic bogs – Lungsmossen (LM), Storhultsmossen (SM), and Björsmossen (BM) – for one year. The electron accepting capacity (EAC) of the retrieved POM decreased by 0.16±0.02 mmol e^-/g dry POM in LM, 0.15±0.02 in SM, and by 0.17±0.01 mmol e^-/g dry POM in BM, as compared with the buried POM_ox, demonstrating extensive electron transfer to the buried POM over the course of one year. Extents of POM_ox reduction were similar for different depths, as tested in BM bog. Exposure of the reduced POM to air (i.e. O₂)resulted in an increase in its EAC, supporting that POM acts as a reversible TEA at the oxic-anoxic interface of peat soils. We complemented these in situ field studies with laboratory incubations of reduced POM collected from the same three bogs. Methanogenic conditions were observed in BM peat soil incubations, which were used for further studies. Amending BM soils with POM_ox and glucose resulted in increases in CO₂:CH₄ formation ratios of several orders of magnitude. These findings pointed towards anaerobic respiration using POM_ox as TEA, thereby suppressing methanogenesis. Taken together, our work provides evidence for POM_ox reduction in situ and substantiates the important role of POM as TEA in controlling CH₄ formation in ombrotrophic bogs.