Coupled Carbon and Nitrogen Cycling in Southern California Biofuel Feedstock Production: Sensor-Based Monitoring to Process Modeling.

Water, carbon, and oxidized nitrogen are elements determining productivity and environmental impact of purpose-grown lignocellulosic feedstocks.  We are investigating the influence of land use conversion to biofuel production under the unusually high temperatures and specific agricultural practices in the low deserts of California. In a pre-plant fallow field and the following initial planting of a high tonnage forage Sorghum, we use an array of soil and tower based sensors with integrated model development to evaluate impacts on the exchange of water vapor, CO₂, NOx.  Our results in the Imperial Valley, CA demonstrate that NOx fluxes from the soil surface are substantially enhanced following application of urea fertilizer, and are inhibited during irrigation events, associated with transient depression of oxygen content and pore gas displacement in these clay soils.  Our results indicate that CO₂ emissions from soil respiration are not inhibited by summer period high surface temperatures (>60°C). Sensor-based measurements are being used to constrain a process model quantifying soil emissions of NOx and CO₂. NO_x efflux is parameterized with respect to fertilizer application, temperature, and oxygen content.  CO₂ efflux is parameterized with respect to labile and recalcitrant carbon content, temperature, and the pulse dynamics of soil moisture and nitrogen.  Sorghum cultivation in the Imperial Valley of CA is expected to be highly productive but also a potentially regionally significant source of NOx, depending on fertilization and irrigation techniques.  Several potentially orthogonal processes, ranging from energy production, net emissions of greenhouse gasses or other climate forcing effects, net emissions of air pollutant precursors, and water use, will determine the overall sustainability of these production systems. This study is quantitatively assessing these impacts of land use conversion to biofuel feedstocks across many of these sustainability metrics, using a combination of measurement and modeling approaches in a highly productive western production environment.