Managing high biomass sorghum for optimum ethanol yield in Missouri

Abstract

High Biomass Sorghum (HBS) has potential for production as a biofuel feedstock in Missouri, but little is known of the crop's yield and appropriate nitrogen (N) management for optimizing ethanol yields, especially in a low-input cropping system on marginal lands. This dissertation is a collection of four studies examining the potential for HBS (Sorghum bicolor (L.) Moench) to be adopted as a biofuel feedstock for ethanol production in the Midwestern U.S. Limited research exists of studying HBS crop yield response to N fertilizer in the Midwest to determine the optimum N rate for maximizing N recovery efficiency (NRE) and N use efficiency (NUE). The first study tested the effects of five N fertilizer rates (0, 56, 112, 168, 224 kg N ha-1 ) on the production of two HBS hybrids (ES 5200 and ES 5201) over two years in central Missouri. Yields of stem and leaf dry matter (DM), and lignocellulosic ethanol (LEY) were measured. Tissue N concentration of leaves and stems were used to calculate N content, NRE, and physiological NUE. Yield of HBS was greatest at 56 kg N ha-1 and above, but NRE and NUE decreased at higher N rates. Reduced rainfall in the second year contributed to no N response in yield. The second study determined the corresponding HBS leaf and stem concentration and contents of 11 macro- and micronutrients from the first study above. Response to N fertilizer rate was controlled by differences between years in rainfall. Reduced DM in the second year resulted in increased concentrations, but less elemental uptake and a resultant delayed N response demonstrate the strong link between nutrient uptake and plant growth following the precipitation. A desire to test HBS yield against other annual bioenergy crops led to the third study, which included establishing long-term research plots at two marginal sites, in central Missouri and southwest Missouri, comparing HBS to maize (Zea mays L.) and sweet sorghum for potential ethanol yield in five years (2010-2014) given minimal inputs. Each crop was planted in a two-year rotation with soybean (Glycine max L.). Theoretical ethanol yields of sweet sorghum and HBS were similar and greater than maize at both locations. Drought severely limited yields of all three crop rotations, but HBS was the most stable in yield across the five years. These results provided a beginning understanding of the stability of HBS on marginal Missouri cropland. Study four involved collecting soil cores from each of the crop plots in study three in 2010, 2011, and 2014 to understand the impact of HBS, compared to maize and sweet sorghum, on soil organic carbon (SOC) concentration and stock, as well as the labile soil carbon fraction (AC). Concentrations of SOC and AC within all crop plots decreased over the first two years at both sites, but after five years the SOC concentrations returned to levels similar to initial 2010 levels, while the AC concentrations decreased. The southwestern Missouri site had a slight positive trend in AC concentration from 2011 to 2014, suggesting beneficial effects from the biofuel feedstocks. Due to the occurrence of drought during the study, the findings have relevance for evaluating land management impacts on SOC in a changing climate. High biomass sorghum is a high-yielding biomass feedstock that shows promise for production in Missouri, especially on marginal lands. Even moderate rates of N fertilizer have the potential to positively increase DM and theoretical ethanol yield in adequate rainfall years. However, intensive nutrient management may prove necessary with continual DM removal. The short-term trend of HBS stemming SOC loss compared to maize production on marginal sites provides support for continued research into the potential for HBS production in Missouri.