Thick ice layers in snow and frozen soil affecting gas emissions from agricultural soils during winter

We investigated soil and snow cover gas concentrations at two agricultural sites (St-Lambert; Chapais) in Quebec, Canada, during winter 1998–1999. Both sites showed frozen and unfrozen soils and complex snow cover structure. At St-Lambert we measured higher average concentrations of N2O (35 to 62 μl L−1) and CO2 (3 to 19 mL L−1) below the frozen soil surface of plots subjected to a treatment of pig slurry than in the control plot (N2O, 9 to 30 μL L−1; CO2, 3 to 7.5 mL L−1). The lack of vertical gaseous concentration gradients in the snowpack was due to the trapping of accumulating gas below the impermeable frozen soil layer. Soil gas concentrations decreased sharply when soil warmed to the freezing point. At the same time, the snow cover was isothermal. N2O could have been lost at spring thaw through gaseous emissions and/or dissolved in meltwaters and leached to the drainage system. High N2O fluxes were measured using closed chambers (215 ng m−2 s−1, slurry treatment; 55 ng m−2 s−1, control) as soon as snow ablation was completed, but became negligible 2 days later, suggesting that emissions were the result of passive degassing rather than of increased biological activity. At Chapais, N2O and CO2 accumulated in the unfrozen soil surface below a thick (0.1 m) basal ice layer. The basal ice layer and the continuous ice layer above it were impermeable to gas diffusion, as demonstrated by the accumulation of a tracer gas (Ar, >50 mL L−1) introduced by a diffuser into the soil. The existence of a basal ice layer is uncommon in eastern Canada. The occurrence of such a phenomenon may increase with climate change due to more frequent rain events during the cold season and affect the dynamics of winter gas emissions from soils.