Sensor-Based Monitoring to Constrain Process Modeling of Carbon and Nitrogen Cycling in Forage Sorghum and Energy Cane Grown As Biofuel Feedstocks in the Low Desert.

Yields of tropical grasses are high under irrigated conditions in low desert, western production areas. These are candidate production systems for purpose-grown biofuel feedstocks. We have tested a range of energy cane and sugarcane genotypes (a range of Saccharum spp. hybrids) and are in early stages of testing a high tonnage biomass Sorghum in the Imperial Valley, CA. We are investigating the influence of land use conversion to biofuel production under the unusually high temperatures and specific agricultural practices in these environments. We have deployed an array of soil and tower based sensors in a pre-plant fallow field and following planting of Sorghum. These 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.  Although yield and energy balance are expected to be favorable, these systems are also potentially significant regionally sources of NOx, depending on fertilization and irrigation techniques. Here we 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.  We also show that CO₂ emissions from soil respiration are not inhibited by summer period high surface temperatures (>60°C). A matrix of factors, including net energy production, net emissions of greenhouse gasses or air pollutant precursors, and other climate forcing effects, as well as 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 this highly productive western production environment.