pH Controls Over Anaerobic Carbon Mineralization to CO2 and CH4 in Peatlands Across An Ombrotrophic-Minerotrophic Gradient.

The efficiency of methane (CH₄) production under anaerobic conditions varies greatly in peatlands across an ombrotrophic-minerotrophic gradient, but the underlying mechanisms that explain this variation are poorly known.  The objectives of this experiment were to determine to what extent differences in soil pH along this gradient control (i) total CH₄ and carbon dioxide (CO₂) production rates and (ii) the efficiency of CH₄ production versus CO₂ production.  We adjusted the pH of peat slurries from an ombrotrophic bog, two poor fens, an intermediate fen, and two rich fens from northern Michigan, USA to 4 levels (3.5, 4.5, 5.5, and 6.5), followed by anaerobic incubation for 43 days.  Increased pH caused a significant increase in CO₂ production in all sites.  Regardless of site, time, and pH level, reduction of inorganic electron acceptors contributed <5% of total CO₂ production.  Higher pH caused acetate pooling by day 7, but this effect was greater in the more ombrotrophic sites and lasted throughout the incubation, whereas acetate was almost entirely consumed as a substrate for acetoclastic methanogenesis by day 43 in the minerotrophic sites. The proportion of acetoclastic vs. hydrogenotrophic methanogenesis increased at low pH, but this effect was only observed in the ombrotrophic sites.  Increased pH greatly enhanced the efficiency of CH₄ vs. CO₂ production by up to 334%.  However, CH₄ production accounted for < 24% of the total gaseous C productions.  Fermentation and/or reduction of humic compounds appeared to be the main pathway for anaerobic C mineralization.  Our results indicate that higher pH increased the fermentative production of acetate and CO₂, but the corresponding increase in acetoclastic methanogenesis in the ombrotrophic sites was insufficient to consume the acetate, suggesting limitation of acetoclastic methanogenesis by some factor in addition to pH in these sites.  pH is an important environmental factor controlling the C mineralization to CO₂ and CH₄ in peatlands, but it alone is insufficient to explain large differences in anaerobic carbon dynamics across the ombrotrophic-minerotrophic gradient.