Bioenergy cropping systems could help offset GHG emissions, but quantifying that offset is complex. Bioenergy crops offset CO₂ emissions by converting atmospheric CO₂ to organic C in crop biomass and soil, but they also emit N₂O and vary in their effects on soil oxidation of methane. Growing the crops requires energy, and so does converting the harvested product to usable fuels. Nitrous oxide and other GHG emissions vary with the climate, soil properties, and land use. The objective of this study was to compare observed and DAYCENT simulated N₂O gas emission rates from bioenergy cropping systems for inclusion in a life cycle assessment quantifying all components contributing to the net GHG emissions. Nitrous oxide was the largest greenhouse gas source. Although DAYCENT did not always capture the observed daily variability in N₂O emissions, it did simulate the observed seasonal patterns within systems and differences in mean emissions among systems reasonably well. Compared with the life cycle of gasoline and diesel, ethanol and biodiesel from the corn-soybean-alfalfa rotation in the near-term reduced net GHG emissions by 64%, reed canarygrass by 119%, and 144% for switchgrass. In the long-term, where soil C sequestration was assumed to no longer occur, a reduction of GHG emissions for the corn rotation was 43%, reed canarygrass 84%, and 114% for switchgrass compared with the life cycle of gasoline and diesel.