title: Life-cycle assessment of net greenhouse-gas flux for bioenergy cropping systems.
abstract: Bioenergy cropping systems could help offset greenhouse gas emissions, but quantifying that offset is complex. Bioenergy crops offset carbon dioxide emissions by converting atmospheric CO2 to organic C 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 (GHG) emissions. We used the DAYCENT biogeochemistry model to assess soil GHG fluxes and biomass yields for corn, soybean, alfalfa, hybrid poplar, reed canarygrass, and switchgrass as bioenergy crops in Pennsylvania, USA. 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 carbon sequestration. N20 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, >200 g CO2e-C x m(-2) x yr(-1) for biomass conversion to ethanol, and >400 g CO2e-C x m(-2) x yr(-1) for biomass gasification for electricity generation. Compared with the life cycle of gasoline and diesel, ethanol and biodiesel from corn rotations reduced GHG emissions by approximately 40%, reed canarygrass by approximately 85%, and switchgrass and hybrid poplar by approximately 115%.
authors.affiliation: Pasture Systems and Watershed Management Research Unit, United States Department of Agriculture-Agricultural Research Service, University Park, Pennsylvania 16802, USA. paul.adler@ars.usda.gov
authors.forename: Paul R
authors.initials: PR
authors.lastname: Adler
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Del Grosso
authors.forename: William J
authors.initials: WJ
authors.lastname: Parton
mesh_terms: D000383:Agriculture / Q000379:methods; D000393:Air Pollutants / Q000032:analysis* / Q000378:metabolism; D018533:Biomass; D002245:Carbon Dioxide / Q000032:analysis* / Q000378:metabolism; D018556:Crops, Agricultural / Q000254:growth & development / Q000378:metabolism*; D004736:Energy-Generating Resources; D000431:Ethanol; D017752:Greenhouse Effect; D000455:Medicago sativa / Q000254:growth & development / Q000378:metabolism; D008897:Panicum / Q000254:growth & development / Q000378:metabolism; D031783:Phalaris / Q000254:growth & development / Q000378:metabolism; D032107:Populus / Q000254:growth & development / Q000378:metabolism; D013025:Glycine max / Q000254:growth & development / Q000378:metabolism; D003313:Zea mays / Q000254:growth & development / Q000378:metabolism
chemical_list: D000393:Air Pollutants; D002245:Carbon Dioxide; D000431:Ethanol