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. Bot h sites showed frozen and unfrozen soils and complex snow cover structure. At St-Lambert we measured higher average concentrations of N2O (35 to 62 mu L L-1) and CO2 (3 to 19 mL L-1) below the frozen soil surface of plots subj ected to a treatment of pig slurry than in the control plot (N2O, 9 to 30 m uL L-1; CO2, 3 to 7.5 mL L-1). The lack of vertical gaseous concentration g radients in the snowpack was due to the trapping of accumulating gas below the impermeable frozen soil layer. Soil gas concentrations decreased sharpl y 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 emi ssions and/or dissolved in meltwaters and leached to the drainage system. H igh N2O fluxes were measured using closed chambers (215 ng m(-2) s(-1), slu rry treatment; 55 ng m(-2) s(-1), control) as soon as snow ablation was com pleted, but became negligible 2 days later, suggesting that emissions were the result of passive degassing rather than of increased biological activit y. 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 ac cumulation 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 t o more frequent rain events during the cold season and affect the dynamics of winter gas emissions from soils.