Partitioning pathways of CO2 production in peatlands with stable carbon isotopes

Although methanogenic pathways generally produce equimolar amounts of carbon dioxide and methane, CO2 concentrations are often reported to be higher than CH4 concentrations in both field and laboratory incubation studies of peat decomposition. In field settings, higher pore water concentrations of CO2 may result from the loss of methane by: (1) ebullition due to the low solubility of methane in pore water and (2) vascular-plant transport. Higher CO2 concentrations may also be caused by: (1) production of additional CO2 by high-molecular weight (HMW) organic matter (OM) fermentation and/or (2) respiration from non-methanogenic pathways. In this study of a peatland where advection and transverse dispersion were the dominant pore water solute transport mechanisms, an isotope-mass balance approach was used to determine the proportions of CO2 formed from non-fractionating OM respiration and HMW fermentation relative to CO2 production from methanogenesis. This approach also allowed us to estimate the loss of CH4 from the belowground system. The pathways of CO2 production varied with depth and surface vegetation type. In a Carex-dominated fen, methane production initially produced 40 % of the total CO2 and then increased to 90–100 % with increasing depth. In a Sphagnum-dominated bog, methanogenesis resulted in 60 % of total CO2 production which increased to 100 % at depth. Both bogs and fens showed 85–100 % of methane loss from pore waters. Our results indicate that the isotopic composition of dissolved CO2 is a powerful indicator to allow partitioning of the processes affecting peat remineralization and methane production.