| Summary: | Sequestering atmospheric carbon in soil is an attractive option for mitigation of rising atmospheric carbon dioxide concentrations through agriculture. Perennial crops are more likely to gain carbon while annual crops are more likely to lose carbon. A pair of eddy covariance towers were set up near Winnipeg Manitoba, Canada to measure carbon flux over adjacent fertilized long-term perennial grass hay fields with high soil organic carbon. In 2009 the forage stand of one field (Treatment) was sprayed with herbicide, cut and bailed; following which cattle manure was applied and the land was tilled. The forage stand in the other field (Control) continued to be cut and bailed. Differences between net ecosystem productivity of the fields were mainly due to gross primary productivity; ecosystem respiration was similar for both fields. When biomass removals and manure applications are included in the carbon balance, the Treatment conversion lost 149 g C m^(-2) and whereas the Control sequestered 96 g C m^(-2), for a net loss of 245 g C m^(-2) over the June to December period (210 days). This suggests that perennial grass converted for annual cropping can lose more carbon than perennial grasses can sequester in a season.
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