Technical Abstract: Life-cycle assessments (LCAs) of bioenergy crops such as switchgrass (Panicum virgatum L.) require data on soil organic carbon (SOC) change and harvested C yields to accurately estimate net greenhouse gas emissions. To date, nearly all information on SOC change under switchgrass has been based on either modeled assumptions or small plot research, both of which do not take into account spatial variability within or across sites for an agro-ecoregion. To address this need, we measured change in SOC and harvested C yield for switchgrass fields on ten farms in the central and northern Great Plains, USA. Change in SOC was determined by collecting multiple soil samples in transects across the fields prior to planting switchgrass and again five years later after switchgrass had been grown and managed as a bioenergy crop. All fields were sampled to a depth of 30 cm, while three fields were sampled to 120 cm. Across sites, SOC increased significantly at 0-30 cm (P = 0.03) and 0-120 cm (P = 0.07), with accrual rates of 1.1 and 2.9 Mg C/ha/yr, respectively, demonstrating the potential of switchgrass as a carbon-negative bioenergy crop within this agro-ecoregion. Change in SOC across sites varied considerably, however, ranging from -0.6 to 4.3 Mg C/ha/yr for the 0-30 cm depth; such variation in SOC change must be taken into consideration in LCAs. Increased reliance on agriculture for energy production underscores the need for long-term environmental monitoring sites in major agro-ecoregions. [GRACEnet Publication]