Modeling year-round CO₂ fluxes and winter subsurface CO₂ dynamics in an Arctic heath ecosystem, West Greenland

Carbon exchange in Arctic ecosystems shows strong seasonality, yet winter processes remain poorly constrained despite their potential importance for annual carbon budgets. In permafrost regions, CO₂ produced in the active layer during late summer and autumn may accumulate beneath frozen soil and snow cover, when gas diffusion to the atmosphere is restricted. Observed wintertime increases in subsurface CO₂ concentrations therefore raise the question of whether they primarily reflect reduced diffusivity or enhanced CO₂ production under relatively warm subnival conditions.

We combined year-round eddy covariance measurements of ecosystem CO₂ exchange, growing-season chamber flux observations, and winter subsurface CO₂ concentration profiles from an Arctic heath ecosystem on Disko Island, West Greenland, to constrain the process-based CoupModel. The model represents soil CO₂ production and transport as functions of soil temperature, moisture, air-filled porosity, and CO₂ concentration, allowing winter physical controls on gas diffusion to be explicitly evaluated.

The calibrated model reproduces observed vertical soil CO₂ concentration patterns between 10 and 80 cm depth as well as the seasonal dynamics of ecosystem CO₂ fluxes. Simulations indicate that elevated winter subsurface CO₂ concentrations are largely explained by reduced gas diffusivity in frozen and snow-covered soils, while the direct influence of high CO₂ concentrations on production rates is limited. Laboratory measurements of CO₂ diffusion under frozen and unfrozen conditions support the strong sensitivity of gas transport to changes in air-filled porosity.

Interannual variability in snow conditions exerts a strong control on non-growing-season CO₂ emissions. Winters with unusually deep snowpacks show substantially higher CO₂ efflux, reducing the annual net CO₂ sink. In contrast, warmer and wetter growing seasons enhance both gross primary production and ecosystem respiration, partially compensating for increased winter losses. These results underline the importance of winter soil physical processes for Arctic carbon dynamics and illustrate how combining observations with process-based modelling can improve estimates of year-round CO₂ exchange.