Limited biofuel feedstock supply?
The United States has embarked on an ambitious program to develop technology and infrastructure to economically and sustainably produce ethanol from biomass. Corn stover, the above-ground material left in fields after corn grain harvest, has been identified as a primary feedstock. Stover and other crop biomass or residue is frequently referred to as "trash" or a waste, implying it has minimal value. However, when returned to the land, this carbon-rich material helps control erosion, replenishes soil organic matter, and improves soil quality. Organic matter in the soil retains and recycles nutrients and improves soil structure, aeration, and water exchange characteristics. In addition, organic matter is the energy source for the soil ecosystem.
"Sustainable biofuel production will require that the functions of organic matter in the soil be addressed before crop residue is removed from the land," states Doug Karlen, USDA-ARS soil scientist at the National Soil Tilth Laboratory at Ames, IA.
Dave Lightle, USDA-NRCS agronomist with the National Soil Survey Center in Lincoln, NE says, "To date, projected sustainable harvest levels have been calculated by reducing total stover production by amounts needed to keep soil erosion losses within accepted limits."
Most estimates of the amount of crop residue that can be sustainably harvested consider only erosion as a constraining factor, without considering the need to maintain soil organic matter. Recently Jane Johnson and her coworkers at the USDA-ARS North Central Soil Conservation Research Laboratory at Morris, MN, reported estimates of the minimum biomass input needed to maintain soil organic matter.
Wally Wilhelm, USDA-ARS scientist with the Agroecosystems Management Research Unit, Lincoln, NE, and his team compared the amount of stover needed to replenish soil organic matter and control water and wind erosion under a limited number of production conditions—continuous corn and corn produced in rotation with soybean with moldboard plow or conservation tillage practices. The amount of stover needed to replenish soil organic matter was greater than that required to control either water or wind erosion in the ten counties (in nine of the top eleven corn production states in the U.S.) investigated. This outcome emphasizes the need to further evaluate the validity of widely circulated estimates of U.S. cropland capacity to sustainably supply feedstock for the emerging cellulosic ethanol industry.
The article appears in the November-December 2007 issue of Agronomy Journal and was the basis of a poster presentation titled "Soil Carbon Needs Limit Biomass Ethanol Feedstock Supply" at the 2007 American Society of Agronomy meetings in New Orleans in November 2007. This research contributes to the USDA-ARS Renewable Energy Assessment Project (REAP) goals and was funded by the USDA-ARS and USDA-NRCS agencies.
The authors conclude that there is a critical need to gather additional high-quality replicated field data from multiple locations to confirm their calculations and to expand the computations to a broader range of cropping systems before major decisions are made about the percent of stover that can designated for biomass energy production. In addition, they state that an extensive effort is needed to expand development of existing crops, discover and develop unconventional crops, and create and deploy advanced cropping systems that exploit the potential of all crops so that biomass production can be greatly expand to provide a sustainable supply of cellulosic feedstock without reducing soil organic matter, thus undermining the productive capacity of the soil.