Constraints on Deep Peat Decomposition: Roles of Redox Conditions, Microbial Communities, and Organic Matter Reactivity

Deep peat is typically more decomposed than shallow peat, and tends to be less available for microbial respiration, producing less methane and carbon dioxide per gram of stored carbon. Understanding why deep peat exhibits slower rates of decomposition – and especially the interplay of redox conditions and organic matter composition – is important for understanding the effects of peat drying and exposure of deep peat at the surface.

To investigate whether the metabolic capabilities of the deep peat microbiome or the reactivity of the peat itself limited breakdown of deep peat organic matter, we conducted a controlled incubation experiment. Incubations were set up to compare deep (>1m deep) and shallow peat (<30cm) from two temperate peatlands (an ombrotrophic, Sphagnum-dominated bog and a minerotrophic, graminoid-dominated fen). Peat samples were incubated under oxic and anoxic conditions, and a subset of vials were inoculated with a shallow microbial community extract, a shallow dissolved organic carbon (DOC) extract or a deep DOC extract from the corresponding site. Headspace gas concentrations (CO₂ and CH₄) were determined over the incubation period, while water samples were taken over the same period to observe changes in DOC concentrations and composition. Microbial community samples were collected at the beginning and end of the incubation period, and 16S rRNA gene sequencing was used to determine shifts in community composition.

We observed that exposure to oxygen and addition of the shallow microbial community increased microbial respiration in comparison to the anoxic deep peat controls. This suggests that the deep peat microbiome is metabolically capable of breaking down deep organic matter, but less efficient, and that without oxygen, the deep peat is less thermodynamically available. However, the amended deep peats do not exhibit CO₂ production rates as high as those in the shallow peat control, indicating that organic matter recalcitrance still governs degradation rates even under aerobic conditions, with implications for the fate of deep peat carbon stocks exposed to oxygen.