Peat Particulate Organic Matter Accepts Electrons During In Situ Incubation in the Anoxic Subsurface of Ombrotrophic Bogs

Abstract

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.