Uncoupling of acetate degradation from methane formation in Alaskan wetlands: Connections to vegetation distribution

Abstract

Laboratory incubations, gas and solute analyses, and stable isotope methods were used to investigate the pathway of methanogenesis in 25 wetland peats of varying vegetation composition along a latitudinal gradient in Alaska. Sites were divided into gross vegetation classes indicative of tropic status: mostly Sphagnum (class 1); Sphagnum plus vascular plants (i.e., Carex) (class 2); mostly vascular plants, but still containing Sphagnum (class 3), and; sites dominated by vascular plants with no visible Sphagnum species (class 4). The magnitude of CO2, acetate and CH4 as end products of anaerobic metabolism varied greatly, but ratios of end product formation indicative of differences in the pathway of C flow and methanogenesis corresponded with vegetation classes, especially at the extremes, e.g., acetate‐C accounted for 67% of total C production in Sphagnum‐rich sites (class 1) decreasing to 13% in sites devoid of sphagna (class 4). Conversely, CH4 comprised only 0.4% of products in class 1 sites, but increased to 14% in class 4. Total respiration rates (sum of all three products) varied by only a factor of ∼2 among vegetation classes (200–440 nmol ml−1 day−1), but rates differed greatly if acetate formation was not included suggesting that belowground C cycling can be much more rapid than previously thought. Apparent fractionation factors (α = δ13CDIC + 1000/δ13CCH4 + 1000) that estimate methanogenic pathway, i.e., the relative contribution of CO2 reduction or acetate as precursors of methane, varied from ∼1.030 to ∼1.080 and agreed with incubation end product ratios, underscoring the importance of the presence or absence of vascular plants and Sphagnum mosses in affecting the pathway of anaerobic C flow. We contend that methanogenesis in general, including CO2 reduction, is impeded in northern wetlands compared to the production of other C compounds and that its importance decreases with oligotrophy. The connection with vegetation suggests that climate change scenarios leading to increases in vascular plant cover in northern wetlands may shift methanogenic pathways toward increased acetotrophy and increased methane formation, which is a positive feedback on warming.