Agriculture Reference
In-Depth Information
A downside of using plants to capture energy from solar radiation is the low effi-
ciency of the photosynthetic process. Only about 0.2% of the energy falling on a
growing plant is captured in the biomass produced (Long et al., 2006). Because photo-
synthetic solar energy capture is low, biomass production, on the scale needed to sig-
nificantly contribute to the transportation fuel supply, will require large land areas.
As with fossil fuel, combustion of biobased fuel forms carbon dioxide, but use of
biofuels is carbon neutral because the carbon emitted when biomass is consumed
is carbon removed from the atmosphere as the plants, which produced the biomass,
were growing. The process will repeat as the carbon is again removed from the
atmosphere as plants grow to produce biomass in subsequent seasons. Thus, the car-
bon cycle for bioenergy is a closed loop with no net addition of carbon to the atmo-
sphere. This is in contrast with carbon flow from use of fossil fuel, in which carbon
sequestered in hydrocarbons formed millions of years ago is released when the fuel
is burned. Unlike the case with bioenergy, there is no process to again recapture the
carbon as fossil fuel, and as a result atmospheric carbon content increases. Therefore,
bioenergy is renewable while fossil energy is not.
Bioenergy is carbon neutral only if it is produced completely without use of fossil
energy. At this time, however, most bioenergy produced is not carbon neutral. That is
because fossil energy (e.g., diesel fuel for tractors and natural gas for nitrogen fertil-
izer) is used in production of crops grown to produce biobased feedstock. In addi-
tion, conversion facilities often rely on fossil fuel (natural gas or coal) for energy to
process biomaterial into biofuels. To be renewable, sustainable, and carbon neutral,
agriculture must eliminate its dependency on fossil fuels.
Biomass for use as energy will likely come from agricultural and forestry enter-
prises. Agricultural biomass useful for energy includes herbaceous crops grown
specifically for energy and residues of herbaceous crops grown for other purposes.
Plant residues include stover from corn and straw from wheat when these crops are
harvested for grain. Forestry biomass includes wood, branches, and other organic
material from trees and shrubs. Additional biomass feedstocks include wastes from
the production, processing, and use of agricultural and forestry materials. These
include animal wastes, wastes from processing of food and nonfood bio-based prod-
ucts, and municipal wastes, including household, lawn and garden, construction, and
demolition materials.
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What infrastructure is available, and what is needed for agriculture to become a
significant source of energy? What are the economic, natural resource, and social
implications, both positive and negative, that energy production would have for agri-
culture and rural areas and for society in general? What are the barriers that must be
overcome to commercialize, on a large scale, bioenergy production and use? These
and other questions must be answered for bioenergy to become viable, commercial,
and sustainable.
