Redox Properties of Peat Particulate Organic Matter: Quantification of Electron Accepting Capacities and Assessment of Electron Transfer Reversibility

Peat particulate organic matter (POM) has been hypothesized to act as a terminal electron acceptor (TEA) for anaerobic microbial respiration in northern peatlands, thereby lowering dissolved methane (CH4) concentrations in these systems. However, the redox properties of peat POM and an assessment of its redox state in situ remain missing, mainly due to the lack of an accurate analytical approach to quantify the number of electrons that are transferred to/from POM. Here, we first developed a new spectrophotometric method to quantify the number of electrons that are transferable to peat POM-denoted hereafter as electron accepting capacity (EAC)-by reacting POM with the reduced species of the probe compound 4,4'-bipyridinium-1,1'bis(2-ethylsulfonate). Second, we used this analytical approach to quantify the EAC of an oxidized reference POM material (0.38 +/- 0.039 mmol e(-)/g POM) and of redox-preserved POM from three ombrotrophic bogs in Varmland, Sweden (0.17-0.24 mmol e(-)/g POM). These EAC values are substantially higher than previous estimates, likely reflecting kinetic artifacts in previous analytical methods. We further established reversible electron transfer to the oxic reference POM over a cycle of electrochemical reduction and O-2 re-oxidation. Finally, we determined that the EAC of two of the three bog POM samples that were collected anoxically (i.e., redox state-preserved POM) increased by similar to 0.09 mmol e(-)/g POM upon exposure to air for 8 days, demonstrating that the POM was indeed reduced in situ. Collectively, our results strongly support that POM acts as a sustainable TEA in northern peatlands, which may lower methane release from these systems.