Growing-season variability and environmental controls of CH₄ and N₂O fluxes across high-latitude terrestrial ecosystems

Abstract. Rapid warming and accelerating ecological change across high‑latitude (>66° N) terrestrial environments are reshaping greenhouse gas dynamics. However, spatiotemporal patterns and environmental drivers of CH₄ and N₂O fluxes in high-latitude ecosystems remain poorly understood.

In this study we describe the seasonal and spatial variation in CH₄ and N₂O fluxes across key subarctic and boreal environmental gradients. A secondary aim is to examine the environmental drivers underlying these flux patterns. We measured fluxes during the snow-free season of 2024 and 2025 in northern Finland, starting early June (spring snowmelt) and ending late September (first snowfall). We conducted 1272 chamber measurements at 144 locations spanning eight high-latitude land cover classes: bare tundra, dwarf-shrub tundra, deciduous forest, evergreen forest, fen, bog, permafrost bog and tundra wetland.

Forest and dry tundra sites acted as net CH₄ sinks (-0.16 to -0.05 mg m^-2 h^-1, depending on land cover class) and as both N₂O sources and sinks (-0.69 to 1.69 µg m^-2 h^-1). Wetlands were net CH₄ sources (0.93 to 27.81 mg m^-2 h^-1) and net N₂O sinks (-0.89 to -6.53 µg m^-2 h^-1). Highest mean CH₄ fluxes occurred in fens (23.4 mg m^-2 h^-1) and permafrost bogs (27.8 mg m^-2 h^-1) which often coincided with substantial N₂O sinks (-4.68 µg m^-2 h^-1). Seasonal trends were present for CH₄ but were land cover‑dependent, with both sinks and sources typically peaking during July–August. Land cover class, soil temperature, and soil pH had the strongest effects on CH₄ fluxes while variation in N₂O was best explained by soil pH and land cover class. However, a significant portion of the variability in N₂O remained unexplained.

These results demonstrate that strong spatial heterogeneity and pronounced seasonal dynamics during the snow‑free period generate substantial variability in CH₄ and N₂O fluxes across high‑latitude ecosystems. Our findings suggest that monthly measurements of in-situ gas fluxes and soil conditions in combination with accurate land cover mapping are essential for modelling and upscaling of GHG fluxes. This is especially important in high‑latitude regions, where rapid warming is altering both ecosystem distributions and soil conditions with significant consequences for current and future GHG budgets.