Geochemical controls on anaerobic organic matter decomposition in a northern peatland

Abstract

The decomposition of deep peat deposits controls the long‐term carbon balance of peatlands but is poorly understood with respect to rates and controls. To rectify this deficiency, we estimated in situ dissolved inorganic carbon (DIC) and methane (CH4) production rates from a beaver pond to a central bog dome and related them to organic matter properties, Gibbs free energies of respiration, and δ13C values of DIC and CH4. DIC and CH4 production decreased from maxima of ~10 nmol cm‐3 d‐1 near the water table to values <0.1 nmol cm‐3 d‐1 at depths >1 m, and there was little differentiation among sites. Deeper into the peat, we measured an accumulation of DIC, CH4, and dissolved organic matter (DOM) enriched in aromatic and phenolic moieties, which resulted from the slowness of diffusive vertical pore‐water movement. Lack of transport may have slowed decomposition in two ways: (1) Aromatic and phenolic DOM moieties accumulated, while the release of carbohydrate‐rich DOM from peat was apparently impeded. (2) The accumulation of DIC and CH4 reduced Gibbs free energy of acetoclastic methanogenesis toward a critical threshold value of ‐25 to ‐20 kJ mol‐1 CH4. Hydrogenotrophic methanogenesis was energetically more favorable and generally dominated according to an isotopic fractionation between CO2 and CH4 of 1.053 to 1.076, but it was apparently impeded by some other factor. We conclude that lateral homogeneity and slowness of decomposition in geologically sealed deep peat deposits are assisted by a lack of solute transport, which facilitates the formation of deep peat deposits over millennia.