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Winter soil CO2 emission is a very important component of the annual carbon budgets, however, almost no information on winter CO2 emission is available from the cropland soil in northeast China. In this study, soil CO2 flux was measured for a 2-year period from an ongoing tillage trial on Black soil in northeast China to quantify seasonal patterns in soil CO2 flux rate and wintertime contribution to annual soil respiration. Average soil CO2 flux rates in the winter (November to March) were between 0.64 to 1.22 g CO2 m-2 d-1, in the non-growing season (October and April) were 2.09-3.56 g CO2 m-2 d-1, whereas in the growing season (May to September) they were between 10.9 to 12.7 g CO2 m-2 d-1, with no significant differences among tillage treatments. Total winter, non-growing and growing season soil CO2 emissions were 0.28-0.45 Mg C ha-1, 0.36-0.53 Mg C ha-1, and 4.52-5.55 Mg C ha-1, respectively, among tillage treatments. The contributions of winter soil respiration to annual soil CO2 emission ranged from 5.1 to 7.1%, and the non-growing season emission ranged from 11.4 to 15.2% among tillage treatments. Our results indicate that in northeast China, cropland Black soil continuously emits CO2 throughout the non-growing season, and the wintertime soil respiration plays a significant role in annual soil carbon budgets. Hence winter soil CO2 emission must be taken into consideration when the role of the soil ecosystem is assessed as either a sink or source of CO2 to the atmosphere. [ABSTRACT FROM AUTHOR]