A principal reason for the use of biofuels and bioenergy is its renewable nature. Utilizing biomass as a biofuel feedstock means that carbon dioxide in the atmosphere that was absorbed and transformed via a plant’s photosyn­thesis is released back into the atmosphere upon combustion of the biofuel. By considering the starting feedstock of the biomass and the end product of biofuels only, without considering any ancillary carbon energy input during the conversion process, the system can be said to be carbon neutral. Carbon neutrality means achieving net zero carbon emission, thus leaving a net zero carbon footprint. Furthermore, by maintaining a balance between plant growth and biomass use, the energy system is both renewable and sustain­able. Sustainability of a feedstock is defined by availability of the feedstock, very positive and beneficial impact on GHG emissions, and no negative impact on biodiversity and land use [9].

There is a growing consensus that carbon dioxide emission and its accu­mulation in the atmosphere is a major culprit of global climate change via human interference with natural cycles of greenhouse gases. Two of the more direct causes for carbon dioxide accumulation in the modern era have been recognized as combustion of fossil fuels and land-use change, in par­ticular, deforestation. The use of biofuels undoubtedly reduces the use of fossil fuels and helps restore the needed balance between the carbon dioxide uptake and release.

Agriculture is also expected to continue to evolve and adapt to new tech­nologies and changing circumstances. Biotechnology is advancing agricul­ture by making available genetically altered varieties of corn and soybeans as well. According to the National Corn Growers Association, biotech hybrids accounted for 40% of the total planted acreage of the United States in 2004 [10]. Crop yields are very important because they directly affect the amount of residue generated as well as the amount of land needed to meet food, feed, and other demands. A joint study sponsored by the U. S. Department of Energy and the U. S. Department of Agriculture offers three scenarios in its report (2005) [10]:

• Scenario 1: Current sustainable availability of biomass feedstocks from agricultural lands

• Scenario 2: Biomass availability through a combination of technol­ogy changes focused on conventional crops only

• Scenario 3: Biomass availability through technology changes in both conventional crops and new perennial crops together with signifi­cant land usage change

Current availability is the baseline case that summarizes sustainable bio­mass resources under current crop yields, tillage practices (20-40% no-till for major crops), residue collection technology (~40% recovery potential), grains to bioethanol and biodiesel production, and use of secondary and tertiary residues [10]. Summing up, the total amount of biomass currently available in the United States as of 2005 for bioenergy and bioproducts was about 194 million dry tons annually. This was about 16% of the 1.2 billion dry tons of plant materials produced on agricultural land of the United States. The single largest source of this biomass potential in 2005 was corn residues or corn stover totaling close to 75 million dry tons [10,11]. Considering that the U. S. corn production was 282 million metric tons in 2005 and 333 million metric tons in 2010, the biomass potential from corn residues alone would have totaled more than 85 million dry tons in 2010. On the other hand, the total biomass derived from forestlands in the United States was estimated to be about 142 million dry tons in 2005 [10]. Therefore, from the standpoints of resource availability and sustainability, intensive R&D efforts focused on efficient conversion of corn residues into biofuels as well as cost-effective conversion of cellulose into ethanol are rationally grounded and well justified.

According to a more recent study conducted and reported in the U. S. Billion-Ton Update [11], for the baseline scenario, projected consumption of currently used resources, the forest residues and wastes, the agricultural res­idues and waste, and energy crops show a total of 1,094 million dry tons in 2030. Under the baseline assumptions, up to 22 million acres of cropland and 41 million acres of pastureland shift into energy crops by 2030 at a simulated farmgate price of $60 per dry ton [11]. This study also shows that the total biomass potential in the United States for the currently used and potential forest and agricultural biomass at $60 per dry ton or less, under the baseline scenario, sharply increases from 473 million in 2012, to 676 million in 2017, to 914 million in 2022, and to 1094 million dry tons in 2030.