Geochemical controls on anaerobic organic matter decomposition in a northern peatland

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 d13C values of DIC and CH4. DIC and CH4 production decreased from maxima of ,10 nmol cm23 d21 near the water table to values ,0.1 nmol cm23 d21 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 225 to 220 kJ mol21 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. Northern peatlands are an integral part of the global carbon cycle. They emit considerable quantities of atmospheric methane (CH4) (Aselmann and Crutzen 1989), discharge dissolved organic carbon (DOC) to adjacent ecosystems (Fraser et al. 2001b), and have accumulated ,450 Pg of carbon over the postglacial period (Gorham 1991) due to a prevalence of primary production over decomposition and soil respiration. The decomposition of peat is thus of considerable scientific interest and has been analyzed by measurements of the atmospheric carbon exchange, laboratory experiments (Yavitt et al. 1997; Bubier et al. 2003), and modeling (Frolking et al. 2001; Belyea and Baird 2006). The majority of organic matter is decomposed in the upper, only seasonally water-saturated layer of peatlands, i.e., the acrotelm, and typically only about 10% of the litter mass reaches the deeper, permanently water-saturated catotelm. In the deep peat layers, decomposition proceeds at perhaps 1% of the rate in the acrotelm (Clymo et al. 1998; Frolking et al. 2001). Several constraints on decomposition processes may occur in deep, water-saturated peat deposits. First of all, low temperatures and a growing recalcitrance of the remaining peat mass slow down respiration (e.g., Yavitt et al. 1997). The poor ability of the deeper peat to decompose may be partially compensated for by an influx of dissolved organic matter (DOM) from the acrotelm with vertical pore-water movement (Siegel et al. 1995), which, for example, occurs with seasonal fluctuations of hydraulic heads in the vertical direction (Waddington and Roulet 1997). In general, however, the catotelm is characterized by a very low hydraulic conductivity (Beckwith et al. 2003), which restricts the significance of such effects. Anaerobic respiration must also be considered as an energetically quite unfavorable process, particularly under geochemical conditions such as those in deep peat deposits (Beer and Blodau 2007). Fermenting and methanogenic populations decompose the organic matter in a stepwise manner under a successive diminution of free energy (Conrad 1999). Methanogenic archaea only utilize the products of fermentation and mostly disproportionate acetate or reduce carbon dioxide (CO2) with hydrogen (H2) to form CH4 and CO2. With CO2 and CH4 accumulating in deep peats, the free energy available to methanogens may decrease to a 1 Corresponding author ([email protected]). Acknowledgments The investigation was in part funded by BMBF (Bundesministerium für Bildung und Forschung, Germany) grant CAN 02/17 and DFG (Deutsche Forschungs Gemeinschaft) grant BL 563/7-1 to C. Blodau and a DAAD (Deutscher Akademischer Austausch Dienst) fellowship to J. Beer. We thank T. R. Moore and N. T. Roulet for access to the Mer Bleue field site and their laboratory, P. Frenzel for measurement of volatile fatty acids, M. Dalva for technical support, and two anonymous reviewers for valuable comments. Infrastructural facilities were supported by the Fluxnet Canada Research Network, which is funded by the Natural Sciences and Engineering Research Council of Canada, BIOCAP Canada, and the Canadian Foundation for Climate and Atmospheric Sciences. Limnol. Oceanogr., 53(4), 2008, 1393–1407 E 2008, by the American Society of Limnology and Oceanography, Inc.