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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.
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