Corn Sustainability for Cellulosic Biofuel and Grain Production in the Southeastern US

Abstract

Biofuel production from plant biomass has been proposed as a solution to mitigate fossil fuel use. Corn (Zea mays L.) is an important crop in the Southeast. Its abundance and high yield potential makes it attractive as bioenergy feedstock for the biofuel industry. The objectives of this study were: 1. develop prediction models that could estimate corn grain and stover yield at harvest using simple measurements at the first reproductive growth stage (R1); 2. determine whether the Neutral Detergent Fiber method (NDF) for extraction and determination of structural carbohydrates can be used as an alternative to the National Renewable Energy Laboratory (NREL) extraction procedure; 3. evaluate the effect of in-season weather conditions, the use of rye (Secale cereale) as a winter cover crop, and the corn residue management on grain yield and biomass yields (total and partial) on two soil types; 4. investigate variations in the distribution of structural carbohydrates and lignin in total biomass and among four plant fractions: above the first ear excluding cobs (top), below the first ear (bottom), cobs alone (cob), and above the first ear including cobs (above-ear); 5. investigate differences in carbohydrates, theoretical ethanol yield (TEY), high heating value (HHV), and mineral content in the total biomass and among the four plant fractions; 6. develop models that predict the total and partial corn stover TEY per unit of area at harvest using only weather conditions in May, June , and July; and 7. investigate the effect of using rye as a cover crop, and corn residue harvest on carbon (C) and nitrogen (N) dynamics on two major soil types of the southeastern US. iii The experiment was established at two locations, one in central and one in north Alabama, in 2009. It consisted of a 3x4x2 complete factorial design arranged in a split-split-plot. Factors were: winter rye cover crop (main plot), nitrogen (N) fertilization rates (sub-plot) and stover residue harvest (sub-sub-plot) replicated three times at each location. A study incorporating stover removal management practices (0 and 100% removal) was also established in South Carolina. Plots in both states were representative of major soil types in their respective region: Alabama plots were Compass and Decatur soils; South Carolina plots were Coxville/Rains-Goldsboro-Lynchburg association. For the development of grain and biomass yield prediction models, the regression was significant with the amount of explainable variability maximized at R1 stage. For the grain yield model, the maximum R2 was 0.7705 and for the stover model maximum R2 reached 0.8473. It seems that total precipitation from planting until R1 growth stage, the amount of N fertilization and simple plant morphological measurements at R1-silking can be used to predict corn grain and stover yield at harvest with some success. A simplified method for carbohydrate analysis was developed. It included the NDF extraction instead of the two-stage extraction proposed by the NREL. There were statistical differences between the two methods in carbohydrate concentrations and TEYs (l kg-1). However, on average the TEYs varied only by 2% which seemed to be practically insignificant. Furthermore, the TEY (l ha-1) prediction derived by the simplified method did not vary from the NREL method. Grain yield ranged from 5,328-9,251 kg ha-1 for the loamy sand and 4,488-6,423 kg ha-1 for the silt loam. Total stover dry weight ranged from 3,486-5,482 kg ha-1 and 3,100-5,528 kg ha- 1 for the same soils. Significant differences in grain and biomass yields were observed between iv individual years and locations, with yields generally greater in central Alabama. For the three years of the experiment, the use of a rye cover crop increased yields in both locations while the average effect of three years of stover harvest was not significant. The use of rye and stover harvest did not affect the concentrations of lignin and structural carbohydrates across plant fractions and soil types. However, their distribution varied greatly among corn plant fractions. Data from this study suggests that in every location the cob, top and above-ear plant portions have the highest holocellulose contents and the lowest lignin contents, which are the most desirable characteristics for bioethanol production. The distribution of glucan, xylan, arabinan, TEYs, HHV, and mineral contents varied significantly among the corn stover portions in every location. However, the use of a rye cover crop and stover harvest had little effect on these variables. Results from this study suggest that harvesting the above-ear portion of the stover would result in a low lignin feedstock with high bioethanol potential and significantly lower nutrient removal rates than removing the total stover. Furthermore, models were successfully developed to predict the total and partial corn stover TEY (l ha-1) at harvest using only the monthly cumulative precipitation and monthly average temperatures in May, June, and July. The R2 values of the models were small to moderate; however, there were not significant differences between the actual and fitted TEYs. A laboratory incubation experiment was performed, in which soil samples were analyzed for total C, N, inorganic N during a 60 days period (0, 30 and 60 days), and CO2-C evolved (30 and 60 days). Carbon and N content in the northern site (1.3% and 0.1%, respectively) were significantly higher than the central site (0.6% and 0.05%, respectively). The use of rye as a winter cover crop did not affect C and N dynamics at either location. For the silt loam in plots where the stover was harvested, C content (1.2%) was significantly lower than plots that stover v was retained (1.4%). In both soil types, N mineralized increased significantly during the 60 day period of the experiment. However, C mineralization did not vary between 30 and 60 days of incubation at either location. Nevertheless, C turnover seemed to be higher in the loamy sand than the silt loam. Results from this study suggest that differences in C and N dynamics due to the use of a rye cover crop and corn stover management are soil dependent. Results from this study indicate that when the objective of an agricultural system is the simultaneous production of biofuel and grain, harvesting only the above-ear portion of the stover can result in high amount of bioethanol across the southeastern US. This would also lead to significantly lower removal rates of C, N, and nutrients when compared to harvesting the whole plant biomass. Furthermore, this study shows that the use of rye as winter cover crop can increase both corn biomass and grain yields. Despite the high C:N ratio of the rye, in this study, the plant available N in the soil did not appear limited and therefore it is recommended that cultivation of winter rye should be incorporated as a management practice in Alabama.