| dc.description.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.
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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
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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
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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. | en_US |
