Influences of the Landscape on Life Cycle Carbon Intensity of Biofuels

Biofuels derived from first (sugar and starch based) and second (lignocellulosic) generation agricultural feedstocks will continue to expand into the market between now and 2022 as incentivized through the federal Energy Independence and Security Act (EISA). Nitrogen use is one of the key environmental concerns within the life cycle since it is both the dominant source of life cycle greenhouse gas (GHG) emissions (energy from N fertilizer production and N2O emissions) and poses risks of reactive N movement throughout agricultural landscapes and watersheds. The other dominant components of the feedstock production on life cycle GHG emissions are tillage and land use change impacts on soil organic carbon (SOC). Opportunities to reduce reactive N through winter double crops may satisfy the dual goal of mitigating N2O emissions and reducing NO3 loses while meeting the objectives of EISA. However, changes in N2O, NO3, and SOC are variable within the agricultural landscape due to soil texture, climate, and crop rotation history thereby increasing the complexity of developing mitigation recommendations. Moreover, the inherent variability in N2O emissions makes it difficult to develop single life cycle carbon intensity profiles for specific fuel pathways that apply across the US, since those pathways will have geographic dependencies. Estimating the expected changes in N2O and SOC is an integral part of quantifying the life cycle GHG profile of biofuels derived from winter double crop feedstocks, while NO3 losses affect both indirect N2O emissions and water quality. The biogeochemical model DayCent was used to simulate the impact of growing winter barley as a double crop following corn before soybean establishment during the winter fallow period for six states in the Mid Atlantic region of the Eastern US on SOC and direct and indirect N2O. EPA is currently reviewing the addition of an advanced fuel pathway for winter barley in the Mid Atlantic region as part of the RFS2 program. Modeling results showed that N2O emissions varied across the landscape; NO3 leaching was higher on the sandy coastal plains soils leading to higher indirect N2O emissions, but direct N2O emissions were higher on the finer textured clay soils. In spite of the variabilities present due to landscape characteristics noted above, when DayCent estimates of changes in N2O emissions and SOC from addition of winter barley into the corn soybean rotation in the Mid Atlantic region were coupled with life cycle inventory results developed for winter barley-to-ethanol fuel expected to be produced from this region, results indicate that the fuel could meet advanced fuel status under EISA because the life cycle carbon intensity of the biofuel is at least 50% lower than a baseline gasoline fuel. This case study demonstrates a possible benefit of one approach to nitrogen management that also meets an important market opportunity mandated by law.