Wintertime CO2 exchange in a boreal agricultural peat soil

We measured the carbon dioxide (CO₂) exchange with the eddy covariance (EC) method through three winters above a cultivated peat soil. During the first winter, the soil was ploughed, while for the next two winters it was grass-covered. On a weekly timescale, the emission was controlled by the soil temperature, whereas the vegetation had no clear impact. The deeper soil temperatures better correlated with the CO₂ efflux, especially in frozen soil. The correlation with the air temperature was poor. After a mid-winter snowmelt, decreased CO₂ efflux rates were temporarily detected, probably resulting from a lowered diffusion of CO₂ from the soil air into the atmosphere. Moderate soil-thaw CO₂ pulses were observed in the springs of ₂001 and 2003. CO₂ emission rates measured with the EC method were found to be significantly lower as compared to those measured with the chamber method. The cumulative CO₂ emission between December and mid-March ranged from 80 to 178 g m^-2 during three winters, correlating positively with air and soil temperatures and the number of snow-free days during that period. The projected increase in the air temperature related to global warming would boost the wintertime CO₂ efflux at our site by 30–200% (35–114 g m_-2), depending on the selected emission scenario.