Seasonal and Diurnal Patterns of Methane Emissions From a Northern Pristine Peatland in the Last Decade

Northern peatlands are key carbon reservoirs and natural sources of methane (CH₄). However, the environmental controls of CH₄-related processes remain unclear, making modeling the emissions a challenge. In this study, we first evaluated the process-based CoupModel with unique long-term (2001–2023) in situ measurements from a pristine sedge-dominated peatland in northern Sweden. Results show that the calibrated model can reproduce the hourly CH₄ fluxes (r² = 0.63) and CO₂ flux, and the abiotic variations well. The CH₄ flux showed significant sensitivity (66% relative importance) to parameters related to CH₄ transport, followed by production and oxidation. We further showed that CH₄ fluxes respond to temperature and water table depth (WTD) with a seasonal hysteresis, suggesting a 35% higher temperature sensitivity during below-average WTD compared to above-average WTD, and a two times higher sensitivity of CH₄ to lowering WTD than to elevating WTD. The hourly growing-season CH₄ fluxes response to temperature also displayed a hysteresis in the diurnal cycle, with nighttime CH₄ fluxes being 14%–23% higher than the daytime fluxes. We presented a CH₄ budget for the site and estimated the annual mean methane emissions from 2014 to 2023 to be 12.2 ± 1.2 gC/m²/yr, identifying the emissions predominantly contributed by diffusion. We conclude that CoupModel can effectively simulate the CH₄ emission and its controls for the northern pristine peatland. Our study reveals the importance of hysteresis in the response of methane fluxes to environmental changes and highlights the need for considering the temporal and hydrologic variability in CH₄-temperature dependencies in peatland management.