We assessed the short-term effects of converting pastureland to hybrid poplar and willow bioenergy plantations on soil greenhouse gas (GHG) fluxes and nitrogen (N) leaching in northern Michigan, USA. We used static chambers to measure soil carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) efflux, and tension lysimeters to measure nitrate (NO3-) leaching, in newly-established poplar and willow plantation plots, and in reference pasture plots. Emissions of N2O increased markedly following cultivation with cumulative direct N2O emissions of 0.3, 4.6 and 5.9 Mg ha(-1) of CO2 equivalents (CO(2)eq) in the reference, willow and poplar plots, respectively. Similarly, land conversion resulted in large increases of NO3 leaching with losses of 2.6, 38.8 and 53.9 kg ha(-1) of N from the reference, willow and poplar plots, respectively. Soil CO2 fluxes were significantly affected by land-use conversion; soils from willow and poplar plots emitted 29-42% less CO2 relative to the reference plots. Greater root respiration in the pastureland likely explained the greater soil CO2 efflux in these plots. Estimates of the net GHG emissions due to land-use conversion were strongly influenced by assumptions regarding the root contribution (RC) to total soil CO2 efflux. Assuming an RC = 50%, we estimate that pastureland conversion at this site incurred GHG debts of 7.4 and 11.6 Mg ha(-1) y(-1) as CO(2)eq for willow and poplar, respectively, during the establishment year. These results demonstrate the need to include soil disturbance impacts on the N cycle in future life cycle assessment of these bioenergy crops. (C) 2012 Elsevier Ltd. All rights reserved.