Arctic and subarctic ecosystems are currently warming faster than other regions of the globe, leading to changes such as seasonal permafrost thaw and the formation of ponds. These ponds emit methane (CH4), a potent greenhouse gas, predominantly through ebullition (bubbling). Microbes in these systems produce CH4 through two primary pathways: acetoclastic and hydrogenotrophic methanogenesis. Stable isotopes can be used to characterize the relative importance of these two pathways for overall CH4 production, providing information that can improve modeling of current and future CH4 emissions.
We used carbon-13 (13C) to determine the dominance of acetoclastic versus hydrogenotrophic methanogenesis in a subarctic peatland located in the discontinuous permafrost region of northern Sweden. Isotopic analysis was performed on porewater samples and captured gas from ebullition collected from seven ponds over six years during the summer. The ponds varied in physical attributes related to their formation such as size and vegetation cover, allowing for identification of the relationship between these attributes and CH4 production pathways. The relative importance of the two CH4 production pathways differed significantly between ponds. Some ponds appear to be dominated by either acetoclastic or hydrogenotrophic production, while others appear to have a mixed methanogen community that remains constant across years. Signatures from porewater and ebullition samples appear to diverge over time in some ponds, indicating a potential increase in oxidation. High between pond variability in δ¬13C-CH4 signatures (-48.9‰ to -93.0‰) highlights the need for further research on the drivers of CH4 production pathways. Minimal literature exists on the types of microbes and metabolic pathways present in subarctic ponds, therefore, conclusions drawn from this unique multi-year study will inform how ponds contribute to the global CH4 budget.
