An assessment of the potential for biomass sorghum to be produced as a dedicated bioenergy crop in Iowa, USA

Abstract

Biomass sorghum (Sorghum bicolor L. Moench) is an annual C4 grass that has gained interest as a dedicated bioenergy crop due to its ability to accumulate large amounts of above-ground biomass even in sub-optimal conditions (i.e. drought). Despite the potential of biomass sorghum as an energy crop, it has not been widely grown in the US as of yet. Corn (Zea mays L.), another annual C4 grass, has been produced on a large scale in the US for biofuels, especially in Midwestern “Corn Belt” states. Iowa is the nation’s top producer of corn, which coincides with the state leading the nation in ethanol production. The high productivity of corn in Iowa calls to question if biomass sorghum could be as or more productive than corn in the state in terms of bioenergy. If this is true, what is the spatial pattern of the energy yield differences and what is the land availability in high-yielding regions? These questions are difficult to answer as prior biomass sorghum field trials in the state have been quite limited in space and time. Efforts have been made to use crop models to fill the gaps left by field experimentation, but have also been limited thus far. To address our research question and build on prior studies, we collect biophysical crop data on biomass sorghum grown in Iowa for two growing seasons, which are used to develop a biomass sorghum plant functional type in an agro-ecosystem model (Agro-IBIS) to predict the potential performance and sustainability of biomass sorghum produced in Iowa.Despite dry conditions in 2019 and severe drought in 2020, biomass sorghum produced ≥20 Mg ha-1 of aboveground biomass in both years. Below the surface, biomass sorghum root samplings revealed that the crop has most of its roots concentrated near the soil surface, with approximately 73% of the root biomass above 50 cm over a 1 m depth. Using the measured field variables (biomass, leaf area index (LAI), canopy height, evapotranspiration (ET), carbon/nitrogen), the parameters of our biomass sorghum model were calibrated to within close agreement with measured biomass (slope = 1.096, R2 = 0.91) and ET (slope = 0.784, R2 = 0.67). Simulated biomass sorghum from our Agro-IBIS model over a 50-year period (1956-2000) revealed a notable north-to-south gradient in biomass yield, with values of >30 Mg ha-1 common in the southern US. Yields in Iowa were near 20 Mg ha-1, close to values measured