Offsetting high water demands with high productivity: Sorghum as a biofuel crop in a high irradiance arid ecosystem

Abstract

High irradiance arid environments are promising, yet understudied, areas for biofuel production. We investigated the productivity and environmental trade-offs of growing sorghum (Sorghum bicolor) as a biofuel feedstock in the low deserts of California (CA). Using a 5.3 ha experimental field in the Imperial Valley, CA, we measured aboveground biomass production and net ecosystem exchange of CO2 and H2O via eddy covariance over three growing periods between February and November 2012. Environmental conditions were extreme, with high irradiance, vapor pressure deficit (VPD), and air temperature throughout the growing season. Air temperature peaked in August with a maximum of 45.7 °C. Sorghum produced an annual aboveground biomass yield of 43.7 Mg per hectare. Net ecosystem exchange (NEE) was highest during the summer growth period and reached a maximum of -68 μmol CO2 m-2 s-1. Water use efficiency, or biomass water ratio (BWR), was high (4.0 g dry biomass kg-1 H2O) despite high seasonal evapotranspiration (1094 kg H2O m-2). The BWR of sorghum surpassed that of many C4 biofuel candidate crops in the United States, as well as that of alfalfa which is currently widely grown in the Imperial Valley. Sorghum also outperformed many US biofuel crops in terms of radiation use efficiency (RUE), achieving 1.5 g dry biomass MJ-1. We found no evidence of saturation of NEE at high levels of photosynthetically active radiation (PAR) (up to 2250 μmol m-2 s-1). In addition, we found no evidence that NEE was inhibited by either high VPD or air temperature during peak photosynthetic phases. The combination of high productivity, high BWR, and high RUE suggests that sorghum is well adapted to this extreme environment. The biomass production rates and efficiency metrics spanning three growing periods provide fundamental data for future Life Cycle Assessments (LCA), which are needed to assess the sustainability of this sorghum biofuel feedstock system.