Paul Adler, USDA-ARS, Building 3702, Curtin Road, University Park, PA 16802, Stephen Del Grosso, USDA ARS NPA SPNR, 2150 Centre Ave, Building D, Suite 100, Fort Collins, CO 80526, and William J. Parton, NREL/CSU, Fort Collins, CO 80523.
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. The objective of this study was to quantify all these factors to determine the net effect of several bioenergy cropping systems on GHG emissions. We used the DAYCENT biogeochemistry model to assess soil GHG fluxes and biomass yields for switchgrass, reed canarygrass, corn, soybean, alfalfa, and hybrid poplar as bioenergy crops in Pennsylvania. DAYCENT results were combined with estimates of fossil fuels used to provide farm inputs and operate agricultural machinery and fossil fuel offsets from biomass yields to calculate net GHG fluxes for each cropping system considered. Displaced fossil fuel was the largest GHG sink followed by soil C sequestration. N2O emissions were the largest GHG source. All cropping systems considered provided net GHG sinks, even when soil C was assumed to reach a new steady state and C sequestration in soil was not counted. Hybrid poplar and switchgrass provided the largest net GHG sinks, greater than 200g CO2e-C·m-2·yr-1 for biomass conversion to ethanol. Compared with the life cycle of gasoline and diesel, ethanol and biodiesel from corn rotations reduced GHG emissions in the near-term by 50-65%, reed canarygrass about 120%, and by about 145 and 165% for switchgrass and hybrid poplar, respectively. In the long-term, where soil C sequestration was assumed to no longer occur, a reduction of GHG emissions for corn rotations was about 40%, reed canarygrass about 85%, and about 115% for switchgrass and hybrid poplar compared with the life cycle of gasoline and diesel.