The Greenhouse Gases, Regulated Emissions, and Energy Use in
the Transportation (GREET) model (v. 2014) was used to determine
field-to-pump GHG emissions for each biofuel conversion scenario.
The GREET model evaluates GHG emissions and energy use from
various domestic and imported U.S. transportation fuels [10]. All
default parameters (GREET v. 2014) were unaltered for the corn
grain, corn stover, and switchgrass biochemical ethanol conversion
pathways with the exception of direct SOC changes measured from
studies in Table 1. Default direct and indirect land use C changes
within the GREET model were not accounted for in this analysis.
Energy use in the agricultural phase consisted of agricultural inputs
(e.g. seed, herbicides, fertilizers), machinery energy use requirements,
and material transport [10]. For corn stover, we allocated
a portion of soil C storage or loss to corn stover based on total
amount harvested (grain and stover) [2]. Greenhouse gas emissions
for corn grain ethanol production were based on a weighted
average from U.S. dry mill and wet mill production. Dry mill production
and wet mill production was estimated to account for
88.6% and 11.4% of U.S. corn grain ethanol production, respectively.
Cellulosic ethanol GHG values from corn stover and switchgrass
were derived from the biochemical pathway in GREET where nonfossil
fuel energy (primarily lignin) is the main energy source for
cellulosic ethanol conversion. The GREET model biofuel GHG parameters
for the agricultural phase are derived on a per unit harvest
mass basis whereas SOC stocks were reported on a per unit area
basis. Soil organic C stocks were converted to C change per harvested
mass basis based on reported grain or biomass yields for
referenced studies [7e9]. Soil organic C stock changes from these
studies (8e10 years) were averaged over a 20-year time frame