Fraser, T. J. and Amiro, B. D. 2013. Initial carbon dynamics of perennial grassland conversion for annual cropping in Manitoba. Can. J. Soil Sci. 93: 379-391. Sequestering atmospheric carbon in agricultural soil is an attractive option for mitigation of rising atmospheric carbon dioxide concentrations. Perennial crops are more likely to gain carbon whereas annual crops are more likely to lose carbon. A pair of eddy covariance towers were set up near Winnipeg Manitoba, Canada, to measure the carbon dioxide flux over adjacent paired perennial grass hay fields with high soil organic carbon. A Treatment field was converted to annual cropping by spraying with herbicide, cutting and tilling. A Control field was cut, but allowed to re-grow. Differences in net ecosystem productivity between the fields were mainly caused by a loss of gross primary productivity in the Treatment field; ecosystem respiration was similar for both fields. When biomass removals and manure applications are included in the carbon budget, the Treatment field lost 149 g C m^-2 whereas the Control field sequestered 96 g C m^-2, for a net difference of 245 g C m^-2 over the June to December period (210 d). This suggests that perennial grass converted for annual cropping can lose more carbon than perennial grassland can sequester in a season.