Why does the efficiency of methane production vary so much among peatlands?

Background/Question/Methods

Wetlands are a major contributor to global CH₄ emissions.  However, the efficiency of CH₄ production (i.e., the ratio of CH₄ to CO₂ production) varies dramatically among peatlands.  Porewater acetate also accumulates in many peatlands despite the fact that it is a methanogenic substrate.  Hypotheses for these phenomena include differences in the availability of labile carbon, concentrations of terminal electron acceptors (TEAs), pH, trace metal deficiencies, and temperature.  We examined the relative importance of these controls over anaerobic carbon cycling (ACC) and CH₄ production in six peatlands that occur along an ombrotrophic-minerotrophic hydrogeomorphic gradient in northern Michigan, USA. 

We measured rates of total anaerobic carbon mineralization, acetoclastic and hydrogenotrophic methanogenesis, and homoacetogenesis under field conditions for six time points over two growing seasons in all six peatlands.  In one time point, we also determined the importance of various inorganic TEAs to ACC.  In a lab experiment, we took peat from the six peatlands and incubated each peat type at pHs from 3.5 to 6.5.  In a second lab experiment, we added anthraquinone-2,6-disulfonate (AQDS), a humic substance-analog, at several temperatures with and without glucose to a bog and fen peat. In both lab experiments, we determined a variety of responses in ACC. 

Results/Conclusions

Ombrotrophic bogs consistently had lower CH₄ production and CH₄ efficiency than more minerotrophic peatlands.  Fermentation reactions dominated ACC with inorganic TEAs being of limited importance.  To our knowledge, we are the first group to measure rates of homoacetogenesis in peatlands.  We demonstrate that this can be an important pathway of acetate production and can compete strongly for H₂ with hydrogenotrophic methanogenesis.

As expected, higher pH enhanced CH₄ production in all sites.  More surprisingly, despite the dominance of the acetoclastic methanogenesis pathway in all sites, acetate concentrations remained high in the ombrotrophic peats at high pH, suggesting the low rates of methanogenesis in these sites cannot be explained solely by their low native pHs.  A separate experiment demonstrated that low trace metal concentrations do not inhibit methanogenesis in these peatlands.  The experiment with AQDS demonstrated that humic substances are highly inhibitory to methanogenesis in ombrotrophic peats but they act like a TEA in more minerotrophic peats.  Our research suggests that the inhibitory effects of low pH and humic substances provide fundamental constraints on CH₄ production in ombrotrophic peatlands, while the low carbon quality of the peat and competition with homoacetogens are important but secondary controls.