Methane concentration and stable isotope distribution as evidence of rhizospheric processes: Comparison of a fen and bog in the glacial Lake Agassiz Peatland complex

This study evaluates relationships between vegetation and stable isotope distribution within a large, northern peat-accumulating wetland. Concentration and δ¹³C for both porewater and emitted methane were obtained from June-September for two systems characterized by different plant assemblages and hydrologic regimes: a Carex-dominated fen and a Sphagnum-dominated, forested bog crest. Average methane emissions were higher in the fen than at the bog crest across the entire growing season. Fen porewater methane concentrations were maintained at consistently low levels in the upper one-third of the peat column, and emitted methane was substantially ¹³C-depleted (by approx. 6‰) relative to shallow porewater methane, trends which are characteristic of passive plant-mediated transport of rhizospheric methane to the atmosphere. Fen porewater δ¹³C-CH₄ values in shallow peat (approx. -59‰) suggest that microbial respiration was primarily driven by acetate fermentation. CO₂ reduction became more important deeper in the peat column with δ¹³C-CH₄ values ranging from approx. -65 to -69‰ between 1 and 2.8 m. In contrast to the fen, porewater methane concentrations in the bog were usually at near-maximum levels just below the water table. δ¹³C values for emitted CH₄ in the bog were enriched relative to those for shallow porewater CH₄ by approx. 10‰, indicating that methane was subject to oxidation as it exited from the peat via passive diffusion. Methanogenesis in the peat at the bog crest appears to have been substrate-limited, with porewater δ¹³C-CH₄ (approx. -67‰) suggestive of CO₂ reduction at all depths. (C) 2000 Annals of Botany Company.