The effects of spatial and temporal variations in acetate and sulfate on methane cycling in two Michigan peatlands

In this 3-yr study, we investigated temporal and spatial changes in pools of pore-water methane, sulfate, and acetate (a methanogenic substrate), along with corresponding methane fluxes. Acetate accumulated to high concentrations (>500 mu M) in the shallow pore waters in winter and spring, indicating that acetogenic bacteria were metabolically active at 2-10 degrees C. High sulfate concentrations in the upper pore waters of one site delayed onset of acetate accumulation, suggesting that acetate consumption by sulfate-reducing bacteria or competition between H-2-consuming sulfate-reducers and autotrophic acetogens caused the delay. Sites dominated by grasses and sedges had larger accumulations of acetate; pore-water pools and fluxes of methane increased rapidly following diminution of the acetate pools at these sites. At a site dominated by Chamaedaphne calyculata, an ericaceous shrub, methane fluxes were lower by 2-3 orders of magnitude; little accumulation of acetate or methane was observed, except in the third year of the study when water levels were consistently high. The seasonal accumulation of acetate and subsequent methane production and flux seem to be dependent on long-term anaerobic conditions in the upper 20 cm of surface peat.