Land management change greatly impacts biofuels’ greenhouse gas emissions
Harvesting corn stover for biofuel production may decrease soil organic carbon (SOC) and increase greenhouse gas (GHG) emissions. Adding additional organic matter into soil or reducing tillage intensity, however, could potentially offset this SOC loss. Here, by using SOC and life cycle analysis (LCA) models, we evaluated the impacts of land management change (LMC), i.e., stover removal, organic matter addition, and tillage on spatially explicit SOC level and biofuels’ overall life-cycle GHG emissions in U.S. corn-soybean production systems. Results indicate that under conventional tillage (CT), 30% stover removal (dry weight) may reduce SOC by 0.04 t C ha-1yr-1 over a 30-year simulation period. Growing a cover crop during the fallow season or applying manure, on the other hand, could add to SOC and further reduce biofuels’ life-cycle GHG emissions. With 30% stover removal in a CT system, cover crop and manure application can increase SOC at the national level by about 0.06 and 0.02 t C ha-1yr-1, respectively, compared to cases without such measures. With contributions from this SOC increase, the life-cycle GHG emissions for stover ethanol are more than 80% lower than those of gasoline, exceeding the U.S. Renewable Fuel Standard mandate of 60% emissions reduction for cellulosic biofuels. Reducing tillage intensity while removing stover could also limit SOC loss or lead to SOC gain, which would lower stover ethanol life-cycle GHG emissions to near or under the mandated 60% reduction. Without these organic matter inputs or reduced tillage intensity, however, the emissions will not meet this mandate. More efforts are still required to further identify key practical LMCs, improve SOC modeling, and accounting for LMCs in biofuel LCAs that incorporate stover removal.