Influence of spatially dependent, modeled soil carbon emission factors on life‐cycle greenhouse gas emissions of corn and cellulosic ethanol

Abstract

Converting land to biofuel feedstock production incurs changes in soil organic carbon (SOC) that can influence biofuel life-cycle greenhouse gas (GHG) emissions. Estimates of these land use change (LUC) and life-cycle GHG emissions affect biofuels' attractiveness and eligibility under a number of renewable fuel policies in the USA and abroad. Modeling was used to refine the spatial resolution and depth extent of domestic estimates of SOC change for land (cropland, cropland pasture, grassland, and forest) conversion scenarios to biofuel crops (corn, corn stover, switchgrass, Miscanthus, poplar, and willow) at the county level in the USA. Results show that in most regions, conversions from cropland and cropland pasture to biofuel crops led to neutral or small levels of SOC sequestration, while conversion of grassland and forest generally caused net SOC loss. SOC change results were incorporated into the Greenhouse Gases, Regulated Emissions, and Energy use in Transportation (GREET) model to assess their influence on life-cycle GHG emissions of corn and cellulosic ethanol. Total LUC GHG emissions (g CO 2 eq MJ À1 ) were 2.1-9.3 for corn-, À0.7 for corn stover-, À3.4 to 12.9 for switchgrass-, and À20.1 to À6.2 for Miscanthus ethanol; these varied with SOC modeling assumptions applied. Extending the soil depth from 30 to 100 cm affected spatially explicit SOC change and overall LUC GHG emissions; however, the influence on LUC GHG emission estimates was less significant in corn and corn stover than cellulosic feedstocks. Total life-cycle GHG emissions (g CO 2 eq MJ À1 , 100 cm) were estimated to be 59-66 for corn ethanol, 14 for stover ethanol, 18-26 for switchgrass ethanol, and À7 to À0.6 for Miscanthus ethanol. The LUC GHG emissions associated with poplar-and willow-derived ethanol may be higher than that for switchgrass ethanol due to lower biomass yield.

Bioenergy production and environmental impacts

“…Among the GHGs, CO 2 and N 2 O are two primary components because of their large quantity and multiapproaches of production (Dunn et al 2013;Qin et al 2016). Theoretically, net CO 2 emissions resulting from the direct use of biofuels are far less than the utilization of fossil fuel, which has been proven by many studies (Dunn et al 2013;Wang et al 2012).…”

Land management change greatly impacts biofuels’ greenhouse gas emissions

“…S2) (Canter et al ., ; Qin et al ., ). It includes the biofuel production stages of feedstock production (including land management), feedstock logistics/storage/transportation, feedstock‐to‐fuel conversion, fuel transportation/distribution, and fuel combustion (Qin et al ., , ). Note that manure collection and storage, which occur off the farm producing corn, were excluded.…”

Well-to-wake analysis of ethanol-to-jet and sugar-to-jet pathways

“…For the sensitivity analysis, the p10 and p90 values of key parameters in corn farming, corn ethanol production, corn stover collection, and corn stover ethanol production shown in Additional file 1: Table A1 were used. As mentioned in “Corn farming, corn stover collection, and ethanol production” section, LUC-related GHG emissions ranges estimated by Qin et al [42] were also examined. Due to lack of reliable range estimates, this study perturbed the other key parameters by ±10% to conduct the sensitivity analysis.…”

“…While the improvements in LUC modeling and assumptions have generally lowered the estimates on LUC-related GHG emissions from the results by Searchinger et al [41], notable variation exists among recent studies depending on LUC models, scenarios, and assumptions (see Additional file 1: Figure A1). Since the LUC-related GHG emissions were not the main focus of this study, this study employed the LUC GHG emissions by Qin et al [42, 43], which documented detailed modeling of LUC and associated GHG emissions of ethanol pathways, including tillage (i.e., conventional, reduced, and no tillage), corn stover removal (i.e., at 0, 30, and 60% removal rates), and organic matter input techniques (i.e., cover crop and manure application). As a baseline assumption, this study used 8 and −0.7 g CO 2 /MJ ethanol for the LUC impacts of corn and corn stover ethanol, respectively, assuming conventional tillage, 30% corn stover removal, and no organic matter input techniques.…”

“…As a baseline assumption, this study used 8 and −0.7 g CO 2 /MJ ethanol for the LUC impacts of corn and corn stover ethanol, respectively, assuming conventional tillage, 30% corn stover removal, and no organic matter input techniques. Acknowledging the variations in the LUC impact, this study also conducted a sensitivity analysis using the ranges of the LUC emissions estimated by Qin et al [42]: 5 to 17 and −1.4 to −0.6 g CO 2 e/MJ for corn and corn stover ethanol, respectively. Note that these ranges do not represent parametric uncertainty rather sensitivity around different scenarios (e.g., tillage types, soil depth, and soil carbon database).…”