High-resolution Methane Isotope Data Improves Model of Wetland Methane Dynamics

Abstract:

Methane flux from wetlands is both a critical component of the global CH₄ budget, and highly sensitive to global climate change. Understanding the variability of this CH₄ source is essential for improving predictions of biosphere-atmosphere feedbacks to global climate. Gaps in our knowledge of the biological mechanisms that underlie CH₄ flux patterns from natural ecosystems limit our ability to scale flux estimates as well as predict future emissions. Process based models, such as the Wetland-Denitrification-Decomposition (DNDC) model used here, are instrumental for testing our understanding of these mechanisms as well as applying them to predict emissions across spatial and temporal scales. Recent incorporation of carbon isotopes into the Wetland-DNDC model has expanded our ability to validate model predictions of the CH₄ production, consumption, and transport processes that yield net emissions. To further improve model estimates we quantified the isotopic composition of CH₄ emissions from a high latitude wetland underlain by discontinuous permafrost (Stordalen Mire, Sweden) and a temperate wetland (Sallie’s Fen, NH). We used quantum cascade laser technology, linked to automated chambers, to measure δ¹³C-CH₄ at a high temporal frequency and partition net CH₄ emissions into its component parts, methanogenesis (including both acetoclastic, and hydrogenotrophic pathways) and methanotrophy (which consumes CH₄ primarily via aerobic metabolism). Comparison of this multi-year dataset to wetland-DNDC model simulations of CH4 flux and δ¹³C highlighted spatial and seasonal drivers associated with vegetation, hydrology and temperature, showing higher fluxes dominated by acetoclastic production in wetter, sedge dominated areas and under warmer conditions. Model predictions of both flux and isotopes most closely matched measured values during peak growing season, with much larger divergence from measurements, especially for δ¹³C-CH₄, during the spring and fall. Together these results provide insights into the role of plant and microbial communities and variable environmental conditions in shaping CH₄ production and consumption patterns in diverse wetland ecosystems as well as highlight the need to focus investigations on the processes underlying CH₄ dynamics during seasonal transitions.