Transport and thermodynamics constrain belowground carbon turnover in a northern peatland

Rates of anaerobic respiration are of central importance for the long-term burial of carbon (C) in peatlands, which are a relevant sink in the global C cycle. To identify constraints on anaerobic peat decomposition, we determined detailed concentration depth profiles of decomposition end-products, i.e. methane (CH4) and dissolved inorganic carbon (DIC), along with concentrations of relevant decomposition intermediates at an ombrotrophic Canadian peat bog. The magnitude of in situ net production rates of DIC and CH4 was estimated by inverse pore-water modeling. Vertical transport in the peat was slow and dominated by diffusion leading to the buildup of DIC and CH4 with depth (5500 mu mol L-1 DIC, 500 mu mol L-1 CH4). Highest DIC and CH4 production rates occurred close to the water table (decomposition constant k(d) similar to 10(-3)-10(-4) a(-1)) or in some distinct zones at depth (k(d) similar to 10(-4) a(-1)). Deeper into the peat, decomposition proceeded very slowly at about k(d) = 10(-7) a(-1). This pattern could be related to thermodynamic and transport constraints. The accumulation of metabolic end-products diminished in situ energy yields of acetoclastic methanogenesis to the threshold for microbially mediated processes (-20 to -25 kJ mol(-1) CH4). The methanogenic precursor acetate also accumulated (150 mu mol L-1). In line with these findings, CH4 was formed by hydrogenotrophic methanogenesis at Gibbs free energies of -35 to -40 kJ mol(-1) CH4. This was indicated by an isotopic fractionation alpha(CO2-CH4) of 1.069-1.079. Fermentative degradation of acetate, propionate and butyrate attained Gibbs free energies close to 0 kJ mol-1 substrate. Although methanogenesis was apparently limited by some other factor in some peat layers, transport and thermodynamic constraints likely impeded respiratory processes in the deeper peat. Constraints on the removal of DIC and CH4 may thus slow decomposition and contribute to the sustained burial of C in northern peatlands. (c) 2007 Elsevier Ltd. All rights reserved.