Sustainability Tradeoffs in Sorghum Bioenergy Production through Coupled Crop- and LCA-Modeling.

Ensuring the sustainability of second generation biofuels requires a comprehensive understanding of productivity–sustainability tradeoffs, many of which occur during feedstock production. Using sorghum-derived E85 fuel grown, processed, and consumed in California's Imperial Valley as our study system, we evaluated productivity-sustainability tradeoffs in feedstock production and fuel product life cycles using coupled crop and life cycle assessment (LCA) models. We investigated 97 management scenarios varying fertilizer strategy (i.e. type and application method) and the intensity of fertilizer- and water-inputs. A subset of scenarios were selected for subsequent LCAs characterizing nine environmental impact categories (EICs). Productivity-sustainability tradeoffs were evident in the production life cycle and were largely driven by fertilizer inputs ( >60% of emissions to all EICs for moderate- to high-N treatments). Accordingly, fertilizer strategies with high nitrogen use efficiency (NUE) (e.g. injected urea, UAN through irrigation) performed much more favorably than those with low NUE (i.e. urea broadcast, unincorporated), with the latter generating 30-50% lower biomass yields and ≥20% higher emissions across ≥7/9 EICs. Across all strategies, emissions increased by up to 20% when N- and water-inputs were unbalanced. When inputs were balanced, increasing N-intensity had mixed effects across fertilizer strategies and EICs. Emissions were generally unaffected when increasing N from low- to moderate-rates, but at the highest N-intensity simulated emissions to air- and water-pollution increased by >25%. These tradeoffs were reflected in the E85 life cycle as feedstock production accounted for >40% of all life cycle emissions across most EICs. Our results suggest moderate, balanced input intensities can produce competitive biomass yields (i.e. 41 Mg ha^-1) while minimizing pollution and reducing water demands. Regardless of management strategy, sorghum-derived E85 had significantly lower greenhouse gas emissions than gasoline, however, it also resulted in substantially higher emissions to air- and water-pollution. These non-target emissions represent important inter-sustainability tradeoffs between fuel-types.