Fire occurs frequently over wetland, but little is known of its impact on soil carbon variations and carbon mineralization, process that are potentially important in global carbon cycle. To investigate this issue, we have designed and implemented a two-year field campaign to quality the effects of fire seasonality and frequency on soil carbon abundance and carbon mineralization in a wetland of the Sanjiang Plain in Northeastern China. A total of 4 burning experiments were conducted over 12 wetland plots from autumn 2007 to spring 2009. Our results show that after burning soil organic carbon (OC) increased in the burned soils during the first two growing seasons. Fire effects on dissolved organic carbon (DOC) and microbial biomass carbon (MBC), however, were more subtle. During the first post-burning growing season, the levels of DOC and MBC were higher than in the unburned soil. The increase however was temporary, and there was no significant difference between the burned and unburned soils in the second growing season. Carbon mineralization rate increased after burning, and CO2 emission rates were higher from burned soils than from unburned soils. Our findings suggest that burning increased CO2 emission to the atmosphere not only during the combustion process, but also through biogeochemical processes in an extended post-burning period.