Winter CO2 emission and its production rate in cold temperate soils of northern Japan: 222Rn as a proxy for the validation of CO2 diffusivity

In the context of a changing seasonal snow-covered period and extent in northern high latitude regions, winter CO2 emissions are vulnerable to drastic climate change, prompting rigorous evaluation of regional carbon budgets. The significance of winter CO2 contributions to the atmosphere in these regions has likewise been recognized. In this study, concentrations of CO2 and 222Rn were observed in the snowpack and underlying cold temperate soils of northern Japan during two winter seasons. Winter CO2 emissions, estimated alternately by snow density, 222Rn tracer (under steady and non-steady states), and chamber, were 56 ± 2, 42 ± 6, 44 ± 15, and 34 ± 3 gCO2-C/m2/season, respectively, suggesting that contribution from average winter CO2 emission corresponds to 25% (range: 19–33%) of annual emission. Winter CO2 emission is highly regulated by soil temperature at 5 cm, depending on snow depth. Winter CO2 emission transport is not suppressed through the snowpack to the atmosphere; rather it is stimulated when seasonal snow depth is over the 40-cm threshold in the soils. The higher soil CO2 production of 0.11 ± 0.02 gCO2-C/m3/h estimated by 222Rn transfer velocity reflects vigorous microbial activity in soils underlying winter snowpack. Therefore, 222Rn is an important proxy for the estimation of winter CO2 emission and diffusivity in snowpack, and of CO2 production rate and transfer velocity in the seasonal, snow-covered