Cellulosic biofuels offer an alternative to grain-based biofuels, which have substantial social and indirect land use costs. However, not all cellulosic biofuels are equal with respect to energy yield and environmental costs of production. We compared farm-level life cycle analysis results for 7 replicated cropping systems in place for 17 years in southwest Michigan: annual crops grown under one of four management types (conventional tillage, no till, low input, and organic management), two perennial crops (alfalfa and poplars), and a fertilized (120 kg N ha^-1y^-1) and non fertilized succesionnal community. For each system we measured fluxes of non-CO₂ greenhouse gases (N₂O, CH₄), soil organic carbon change, and the carbon costs of different agronomic practices, together with biomass yields, to produce a measure of global warming impact for each crop. Additionally, we calculate for each crop an environmental sustainability factor, describing the ratio between carbon costs and energy yields for each system.

Results/Conclusions

The poplar plantation had the lowest net carbon cost (4.4 g CO₂ m^-2 y^-1) but also the lowest potential fuel production. In contrast, the fertilized, early successional system provided the most carbon mitigation with a net global warming impact of -714 ± 68 gCO₂ m^-2 y^-1 for a fuel yield of 2919 ± 321 L ha^-1. Land requirements of the successional system were  5% larger than for alfalfa (Medicago sativa), the most productive system in our analysis. The alfalfa system had the largest biofuel production rates of 3022 ± 67 L ha^-1 y^-1 but this high productivity was accompanied by a larger net global warming impact of -412 ± 52 gCO₂ m^-2 y^-1.

By simple extrapolation, alfalfa for biofuel feedstock production would require about 21.0 × 10⁶ ha of arable land (or ~16% of total arable land in U.S.) in order to reach the U.S. legislative mandate for 16 billion gallons of ethanol per year from renewable sources by 2022. Successional vegetation would require 10% more land area but provide almost twice the carbon mitigation with no indirect carbon cost if grown on marginal land not now used for food production.