Peat particulate organic matter accepts electrons during in situ incubation in the anoxic subsurface of ombrotrophic bogs

Peat particulate organic matter (POM) in the anoxic subsurface of peatlands is increasingly recognized as an important terminal electron acceptor (TEA) in anaerobic respiration.While POM reduction has been demonstrated in laboratory peat-soil incubations and (electro-) chemical reduction assays, direct demonstration of POM reduction in peat soils under in situ, field conditions involving quantification of transferred electrons remain missing.Herein, we demonstrate that deployment of an oxidized reference POM in the anoxic, methanogenic subsurface of three ombrotrophic bogs, followed by one year incubation, resulted in the transfer of approximately 150 -170 μmol of electrons per gram POM to the deployed reference POM.The capacity of this reduced POM to accept electrons was partially restored upon subsequent exposure to dissolved oxygen.These findings provide direct evidence for POM acting as regenerable and sustainable TEA for anaerobic respiration in temporarily anoxic parts of peat soils.Based on the number of electrons transferred to POM and thermodynamic considerations, we estimate that anaerobic respiration to POM may largely suppress methanogenesis in peat soils, particularly close to the oxic-anoxic interface across which POM is expected to undergo redox cycling. Plain language summaryIn oxygen-depleted peat soil, microbes slowly break down peat material to CO 2 in a series of processes summarized under the term anaerobic respiration.These processes have in common that they require a chemical substance to which microbes can transfer electrons liberated in anaerobic respiration.Previous studies provided evidence that peat particulate organic matter (POM), the organic material primarily making up peat soils, can act as this electron acceptor.In this work, we buried a reference POM, placed in small mesh bags, in three Swedish peat soils.After one year, analysis of the recollected POM indeed showed that it had taken up electrons.Some of these electrons could be subsequently removed from the POM by bringing it in contact with dissolved oxygen.Taken together, our study supports that POM in peatlands accepts electrons, temporarily stores them, and then transfers them to oxygen when it becomes available.In this role of electron acceptor, POM may significantly decrease the formation of large amounts of methane, a gas that, when released from peatlands, largely contributes to warming of the atmosphere.Electron transfer to POM in peatland soils thus may play an important and desirable role in lowering natural methane release from peatland ecosystems. Key points Oxidized reference peat particulate organic matter (POM) was reduced in the anoxic subsurface of three ombrotrophic bogs over one year. On average, 160 mol electrons were transferred per gram reference POM across the three bogs. The reduced POM was partially re-oxidized by dissolved oxygen, supporting that POM is a sustainable terminal electron acceptor.