Land conversion to bioenergy systems is responsible for major increase in atmospheric CO₂ concentration over the past 150 years. The Eddy Covariance (EC) technique was used to examine CO₂ fluxes at seven sites in southwestern Michigan undergoing such conversions. Of the seven sites, four had been managed for the Conservation Reserve Program (CRP) during the last 20 years to maintain them as grasslands dominated by smooth brome (Bromus inermis Leyss). The other three were cultivated in a corn/soybean rotation. The effects of land use change were studied during 2009 when six of the fields (three CRP and three crop fields) were converted to soybean cultivation, with the 7^th site remained as a grassland reference. In early May, herbicide (Glyphosate) was applied to kill the vegetation before planting all six sites to soybean. Soybean was planted to suppress the brome grass and ‘homogenize’ the fields prior to their conversion to cellulosic biofuel cropping systems in 2010.

Results/Conclusions During winter the corn fields were essentially carbon (C) neutral while the grasslands were C sources, with average emissions of 15 g C m^-2 month^-1. In April 2009, while the corn fields continued to be a source of C, the grassland sites switched from a C source to a sink. Two days after herbicide application, the grassland sites switched back to a source. After planting the soybean in mid June, all sites continued to be C sources until the end of June. In July, the sites previously cultivated with corn became C sinks, accumulating 15 to 50 g C m^-2 month^-1. Conversely, the sites converted from grassland to soybean continued to be net sources of C. The annual NEP at sites converted from CRP to soybeans had a net emission ranging from 156 (± 25) to 128 (± 27) g C m^-2 year^-1. The sites previously cultivated as corn/soybean rotation was a net C uptake ranging from -91 (± 26) to -57 (± 21) g C m^-2 year^-1 although at one site, characterized by a lower soil C content and lower biomass production compared to the others, the NEP was 31 (± 10) g C m^-2 year^-1. The conversion of CRP to soybean cultivation in preparation for biofuel crop production induced net C emissions.