Life Cycle Assessment of Sorghum Cultivation As Biofuel Feedstock in an High Temperature Environment.

As California strives to increase production of domestic renewable energy, it is increasingly important that the costs and benefits of potential avenues for in-state energy production be evaluated quantitatively. The extreme irradiance (>3000 MJ photosynthetically active radiation (PAR) m⁻² yr⁻¹) of California’s desert ecosystems presents a unique opportunity for bioenergy production. Sorghum bicolor is a heat tolerant, re-sprouting C₄ species with high water and radiation use efficiencies (4.0 g dry biomass kg-¹ H₂O and 1.5 g dry biomass MJ^-1, respectively), making it a promising candidate for bioenergy production in the high heat, high light, arid environment of southern California’s Imperial Valley, provided it receives regular irrigation. Indeed, in our 5.3 ha plot, S. bicolor achieved notably high assimilation rates (i.e. A_max= 58 µmol m^-2 s^-1) with sustained productivity despite extreme temperatures (i.e. peak temperature of 45.7⁰ C), and competitive yields of 58 tons dry weight ha^-1 year^-1. Yields of this magnitude are capable of generating as much as 11,362 l ha^-1 ethanol through cellulosic ethanol production. This figure, however, does not provide a full measure of the net costs and benefits and overall sustainability of such a biofuel production system. For example, the environmental costs of using fuel-dependent machinery, synthetically generated fertilizers and pesticides, product transportation, and conversion processes occur at quantifiable carbon costs to the production system. The investment of limited water resources for irrigation also represents an important ecological tradeoff within the Imperial Valley’s agricultural region. In order to better assess the net environmental costs and benefits, we have developed a detailed model on select environmental metrics. This systematic LCA approach allows us to incorporate additional externalities of regional air pollution and water use, that are necessary for biofuel sustainability and for compliance with state and federal renewable/low carbon fuel policy.