Seasonal trends of soil CO2 dynamics in a soil subject to freezing

Seasonal trends and controls of soil CO2 concentrations are important for understanding soil carbon cycling, soil acidification and CO2 emissions to the atmosphere. This is particularly the case for cold region soils, since these hold extensive soil carbon reserves and are often subject to fluctuating environmental conditions. This paper evaluates and simulates seasonal trends and controls of subsurface CO2 dynamics at a tundra-heath site in NE-Greenland. The study consisted of field measurements of soil CO2 efflux, temperature, water content, pore gas composition in soil profiles as well as temperature- and moisturecontrolled laboratory experiments. Diurnal and seasonal variations in observed CO2 effluxes correlated fairly well with nearsurface temperatures (r2>0.8) except during periods where soil CO2 efflux was dominated by CO2 being released in burst due to rapid near-surface thawing or major precipitation events. Lack of correlation in these situations is considered a result of non-steady state conditions. Laboratory experiments on soil samples collected from four horizons revealed that the effect of temperature and water content on soil microbial respiration can be modelled by simple fit equations which explained 95% of the variation in observed soil CO2 effluxes during the 2001 growing season. Using observations of the water content and subsurface CO2 concentration with depth and time it was possible to predict the depth-dependent CO2 production using a steady state diffusion model (PROFILE). The resulting simulated CO2 production profile and soil CO2 effluxes agreed with observations and revealed the importance of CO2 diffusion for understanding subsurface soil CO2 dynamics. In addition, seasonal rates of soil CO2 production were predicted using the verified fit equations and observed soil temperatures and water contents. This final model leads to a discussion on shifts in factors controlling of subsurface CO2 dynamics over the year as well as the importance of such shifts in relation to future field and modelling studies of soil respiration dynamics in soils subject to freezing.