Initial greenhouse gas emissions and nitrogen leaching losses associated with converting pastureland to short-rotation woody bioenergy crops in northern Michigan, USA

摘要

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 (CO_2), nitrous oxide (N_2O) and methane (CH_4) efflux, and tension lysimeters to measure nitrate (NO_3) leaching, in newly-established poplar and willow plantation plots, and in reference pasture plots. Emissions of N_2O increased markedly following cultivation with cumulative direct N_2O emissions of 0.3,4.6 and 5.9 Mg ha ~1 of CO_2 equivalents (CO_2eq) in the reference, willow and poplar plots, respectively. Similarly, land conversion resulted in large increases of NO_3 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 CO_2 fluxes were significantly affected by land-use conversion; soils from willow and poplar plots emitted 29-42% less CO_2 relative to the reference plots. Greater root respiration in the pastureland likely explained the greater soil CO_2 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 CO_2 efflux. Assuming an RC = 50%, we estimate that pastureland conversion at this site incurred GHG debts of 7.4 and 11.6 Mg ha ~1y ~1 as CO_2eq 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.