Redox Properties of Peat Particulate Organic Matter From Five Ombrotrophic Bogs in Central Sweden

Peat particulate organic matter (POM) is increasingly recognized as an important terminal electron acceptor (TEA) for anaerobic microbial respiration in anoxic peat soils. The goal of this work was to quantify the electron-accepting capacity (EAC) of POM that is accessible to microbes in these soils under in situ conditions. To this end, we collected 28 reduced POM samples from the anoxic subsurface along transects in Sphagnum-dominated ombrotrophic bogs in central Sweden. These POM samples had similar physicochemical properties and compositions within and across peatlands, as inferred from elemental analysis, infrared spectroscopy, and solid-state ¹³C nuclear magnetic resonance spectroscopy. The microbially accessible EAC of these POM samples were determined by quantifying the increases in the total EACs and concomitant decreases in electron-donating capacities when reacting the field-collected reduced POM with dissolved oxygen (DO). These analyses suggested that between 90 and 390 μmol electrons per gram of POM carbon are microbially transferrable to POM. The reaction of POM with DO was found to also result in equimolar conversion of electron-donating to electron-accepting moieties in POM, demonstrating fully reversible electron transfer to and from POM and, therefore, that POM is a sustainable TEA in temporarily anoxic peat soils. A comparison of the microbially accessible EAC of POM to that of pore-water inorganic TEA species and reported EACs and measured concentrations of peat-dissolved organic matter revealed that POM is the major TEA in the studied bogs.