**8. Biofuels production and crop rotations**

Interest in using lignocellulosic biomass to produce ethanol is gaining in popularity. Lignocellulosic biomass production mainly involves growing and harvesting plants generally not used for food or feed. Woody species such as willow (*Salix*. spp.) and poplars (*Populus* spp.) (Matthew et al., 2010) and grasses such as switchgrass (*Panicum virgatum*), big bluestem (*Andropogon gerardii),* reed canarygrass (*Phalaris arundinacea*) are several sources of lignocellulosic biomass that have shown to be useful in ethanol production (Hill, 2007). Rotation schemes for growing lignocellulocsic crops are, for the most part, still in development. Production of these materials for biofuels though is being done mostly on land not suitable for extensive corn and soybean production thus relieving pressure to grow more hectares of these crops to satisfy the conventional and biofuels markets. Worldwide it is estimated that about 1% of crop land or about 11-12 million hectares are being used to grow biofuels (de Fraiture et al., 2008). Raghu et al., (2006) points out that some of the traits favorable to producing a lignocellulosic crop such as being a C4 photosynthetically, lacking pests, rapid early season growth, and long canopy duration can also tend towards them being invasive. This would not work well in a rotation scheme with most conventional crops. Currently lignocellulosic crops contribute little to current U.S. transportation biofuel suppliers but will likely provide the great share of ethanol in the near future.

In the United States, debate is underway concerning the use of corn as a primary source of fuel ethanol. Also, soybean oil is being blended with diesel to extend it. Diversion of these crops for biofuels is believed to increase food prices for consumers as the competing interest of food and fuel vie for the available supply. There is also concern that the increased demands for these grains will negatively impact crop rotations, particularly those that help conserve soil, water, and plant nutrients. One of the primary reasons corn and soybean are currently popular for biofuels over lignocellulosic crops is the comparatively short time to harvest maturity. Most of the lignocellulosic crops require three to five years to reach harvest maturity(Hill, 2007), compared to one year for corn or soybean. Though corn and soybean are regularly rotated with one another, history has shown that a substantial increase in the price received for any crop can encourage monocultures at the expense of proven rotations and their accompanying benefits. The expanded use of corn and soybean as biofuel could diminish the inclusion of small grains and/or forage crops in rotation schemes and the tillage of soils that are not well suited for cultivation.

Besides lignocellulosic crops, the harvesting of crop residues for ethanol production has been considered. Perlack et al., (2006) has reported that nearly 7.0 X 1010 kg of corn stover

al. (1986) also reported from this research at other locations that conservation tillage systems resulted in increased soil water contents, lower soil temperatures, increased soil organic matter, and more water-stable aggregates near the soil surface with higher bulk densities than conventional tillage. Corn yields were found to be equal to or better than conventional tillage practices when grown on the better drained soils using conservation tillage. Only on the poorly drained soils did corn yields on conservation tilled fields fail to exceed conventional tillage, most likely due to low temperatures and excess wetness in the spring

Tillage Previous crop Corn Soybean Soybean Corn System Crop Corn Corn Soybean Soybean Fall plow 10.7 11.6 3.6 3.8 Fall chisel 10.3 11.4 3.3 3.6 Ridge till 10.4 11.6 3.4 3.6 No-till 9.1 11.2 3.2 3.3 Table 7. Mean corn and soybean yields (Mg ha-1) in response to tillage system and crop rotation on a Chalmers silty clay loam in Indiana in 1980-1984 (data is from 6th to 10th year of

Roth (1996) presents several crop rotation schemes to use in no-till farming on Pennsylvania dairy farms. One of the more popular is an alfalfa-grass sward for hay followed by no-till corn. This involves killing the sod in the fall with herbicides to control weeds and reduce residue by early spring to facilitate corn planting. This rotation seems to work best where hay production is limited to three years. Alfalfa is also successfully no-tilled into fields that have just been harvested for corn silage or following the harvest of a spring seeded sorghum sudangrass. Lafond et al., (1992), evaluated no-till, minimum till (one pre-seeding tillage operation) and conventional till (fall and spring pre-seeding tillage operations) on a fouryear crop rotation study. The rotations were fallow-spring wheat- spring wheat-winter wheat, spring wheat-spring wheat-flax (*Linum usitartissimum* L.)-winter wheat, and spring wheat-flax-winter wheat- field pea. Tillage systems did not affect the amount of water conserved during fallow. However, no-till and minimum till did result in an increase in soil water from the surface to 120 cm in depth over conventional till. All three crops in the study had greater yields in the no-till and minimum till treatments than in the conventional till. In an experiment using conservation tillage practices (strip-till or no-till) in combination with a corn-soybean rotation, both full-season soybean or double-crop soybean following wheat

In recent years there has been considerable interest in various tillage practices and their influence on the sequestration of atmospheric CO2 as a partial means of mitigating its current increase and subsequent impact on climate change. Sampson and Scholes (2000), state that the optimization of crop management to facilitate accumulation of soil organic matter could help sequester atmospheric CO2 and lower the rate of its increase. West and Post (2002), found that, excluding a change to no-till in wheat-fallow rotations, a change from conventional tillage to no-till can sequester between 43 to 71 g C m-2 yr-1. These values are within the upper range (10 to 60 g C m2 yr-1) of those reported in a review by Follet (2001). West and Post (2002) also stated that enhanced crop rotation complexity can sequester an average of 8 to 32 g C m-2 yr-1 which is similar to an average of 20 g C m-2 yr-1

had the most consistent increase in seed yields (Edwards et al., 1987).

as depicted with the Chalmers silty clay in Table 7.

the study). (Kladivko et al., 1986).

could be harvested in the U.S. for ethanol production. Worldwide other crops that produce sufficient quantities of residue that could be used to produce ethanol include rice, barley, oat, wheat, sorghum, and sugar cane (*Saccharum officinarum* L.). The use of crop residues for lignocellulosic ethanol production has however run into opposition due to the negative impacts such removals have on C sequestration, soil properties, and nutrient availability for subsequent crops. Wilhelm et al., (2007) reported that between 5.25 and 12.50 Mg ha-1 of corn stover are required to maintain soil C at productive levels for subsequent crops. Lal (2004) states that even though the energy acquired from the world's crop residue would be equivalent to 7.5 billion barrels of diesel, a 30% to 40% removal of crop residue would increase soil erosion and its subsequent pollution hazards, deplete soil organic C, and increase CO2 and other greenhouse gas emissions from the soil. He suggests establishing biofuel plantations of adapted species on marginal lands rather than remove crop residues from land used to grow food and feed grains. Development of such plantations will require more aggressive research into developing crop rotation schemes specific for growing lignocellulosic crops for biofuel.
