pH controls over anaerobic carbon mineralization, the efficiency of methane production, and methanogenic pathways in peatlands across an ombrotrophic-minerotrophic gradient

Methane (CH4) production varies greatly among different types of peatlands along an ombrotrophic-minerotrophic hydrogeomorphic gradient. pH is thought to be a dominant control over observed differences in CH4 production across sites, and previous pH manipulation experiments have verified the inhibitory effect of low pH on CH4 production. In this experiment, we asked (i) if the major effect of low pH is direct inhibition of one or both pathways of methanogenesis and/or inhibition of 'upstream' fermentation that provides substrates for methanogens, and (ii) to what extent is pH sufficient to explain differences in CH4 production relative to other factors that co-vary across the gradient. To address these questions, we adjusted the pH of peat slurries from 6 peatlands to 4 levels (3.5, 4.5, 5.5, and 6.5) that reflected their range of native pH, maintained these pH levels over a 43-day anaerobic laboratory incubation, and measured a suite of responses within the anaerobic carbon cycle. Higher pH caused a significant increase in CO2 production in all sites. Regardless of site, time, and pH level, the reduction of inorganic electron acceptors contributed to <12% 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 completely consumed as a substrate for acetoclastic methanogenesis by Day 43 in the minerotrophic sites. Higher pH also enhanced CH4 production and this process was up to 436% more sensitive to changes in pH than CO2 production. However, across all sites and pH levels, CH4 production accounted for <25% of the total gaseous C production. Fermentation appeared to be the main pathway for anaerobic C mineralization. Our results indicate that low pH inhibits CH4 production through direct inhibition of both methanogenesis pathways and indirectly through its effects on fermentation, but the direct effects are stronger. The inability of acetoclastic methanogenesis to fully compensate for acetate pooling in ombrotrophic peats at higher pH suggests that CH4 production is inhibited by some factor(s) in addition to pH in these sites. We examine a variety of other potential inhibitory mechanisms and postulate that humic substances may provide an important inhibitory effect over CH4 production in ombrotrophic peatlands. (C) 2012 Elsevier Ltd. All rights reserved.