Relationship between Trophic Status and Methanogenic Pathways in Alaskan Peatlands

Abstract:

To improve predictions of naturally emitted CH₄ from northern wetlands, it is necessary to further examine the methanogenic pathways in these wetlands. Stable isotope C ratios (δ¹³C) have been used as a robust tool to distinguish different pathways, but different sources of parent compounds (acetate and CO₂) with unique δ¹³C may add complexity to previously established criteria. Large portions of peatlands accommodate a mixture of different sphagna and sedges. Plant species may look very similar and belong to the same genus but are different morphologically and physiologically. To better understand the relationships between surface vegetation patterns and methanogenic pathways, 26 peatland sites were studied in Fairbanks and Anchorage, Alaska in summers of 2014 and 2015. These sites were ordinated using multiple factor analysis into 3 clusters based on pH, temp, CH₄ and volatile fatty acids production rates, δ¹³C values, and surface vegetation species/pattern. In the low-pH trophic cluster (pH~3.5), non-vascular/vascular plant ratios (NV/V) were ~ 0.87 and dominated by diverse Sphagnum species and specific sedges (Eriophorum vaginatum), and fermentation was the dominant end-point in decomposition with no CH₄ detected. Although NV/V is about the same in the intermediate cluster (0.74) (pH~4.5), and Sphagnum squarrosum was largely present, both hydrogenotrophic (HM) and acetoclastic methanogenesis (AM) were very active. Syntrophy was present at certain sites, which may provide CO₂ with unique δ¹³C for CH₄ production. At the highest pH trophic cluster examined in this study (pH~5), non-vascular plants were almost not existent and Carex aquatilis dominated. CH₄ production rates (mainly HM) were slower than those in the intermediate cluster and the apparent fractionation factor a was lower than in the sites with syntrophy, which warrants further investigation of the position and compound specific δ¹³C analysis of volatile fatty acids.