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Life Cycle Assessment of Net Greenhouse Gas Flux for Bioenergy Cropping Systems

Steve Del Grosso*
USDA ARS NPA SPNR, 2150 Centre Ave., Building D, Suite 100, Fort Collins, CO 80526

Paul Adler
USDA-ARS-Pasture Systems and Watershed Management Research Unit,
Bldg. 3702 Curtin Road, University Park, PA 16807

Bill Parton
NREL-CSU, 1231 East Drive, Fort Collins, CO 80523

Sadie Skiles
USDA ARS NPA SPNR, 2150 Centre Ave., Building D, Suite 100, Fort Collins, CO 80526


Bioenergy cropping systems could help offset greenhouse gas emissions, but quantifying that offset is complex. Bioenergy crops offset carbon dioxide emissions by converting atmospheric carbon dioxide to organic carbon in crop biomass and soil, but they also emit nitrous oxide and vary in their effects on soil oxidation of methane. Growing the crops requires energy (e.g., to operate farm machinery, produce inputs such as fertilizer), and so does converting the harvested product to usable fuels (feedstock conversion efficiency). The objective of this study was to quantify all these factors to determine the net effect of several bioenergy cropping systems on greenhouse gas emissions. We used the DAYCENT biogeochemistry model to assess soil greenhouse gas fluxes and biomass yields for corn, soybean, alfalfa, hybrid poplar, reed canarygrass, and switchgrass 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 greenhouse gas fluxes for each cropping system considered. Displaced fossil fuel was the largest greenhouse gas sink followed by soil carbon sequestration. N2O emissions were the largest greenhouse gas source. All cropping systems considered provided net greenhouse gas sinks when the benefits of co-products were included. Compared with the life cycle of gasoline and diesel, ethanol and biodiesel from corn rotations reduced greenhouse gas emissions by about 40%, reed canarygrass by about 85%, and by about 115% for switchgrass and hybrid poplar.