Scientists see limited supply of corn stover for cellulosic biofuels in the US, urge more research into new biomass crops

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 one of the primary potential feedstocks. But scientists from the USDA's Agricultural Research Service (ARS) say part of this 'trash' or a waste needs to be returned to the land to control erosion, replenish soil organic matter, and improve soil quality. They critically evaluate the amount of corn stover that would be available for biofuels under such optimal conditions and conclude that more comprehensive assessments are needed. At the same time, they urge more research into new biomass crops and cropping systems to expand the biomass supply while maintaining soil organic matter.

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. 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 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:
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Sustainable biofuel production will require that the functions of organic matter in the soil be addressed before crop residue is removed from the land, stated 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 added that 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.

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.

Biopact would, with some care, deduce that this research may indicate that the genuinely sustainable biomass supplty for next-generation biofuels in the U.S. is more limited than often estimated. If all corn stover were to be left in the field, the US would have to expand its arable land base to grow dedicated energy crops. But the US (like Europe) already utilizes most of its potential arable land: out of 354 million hectares of land estimated to be 'potentially arable' (FAO Terrastat), around 269 million hectares are seen as 'equivalent arable land' (land that can actually be converted to grow crops). Of this amount, the US already utilizes around 175 million hectares (latest data from the FAO Aquastat database).

In short, the scope for dedicated biomass production in the US appears to be rather limited. This would be in line with assessments by scientists from the International Energy Agency's Bioenergy task forces, who estimate North America's total sustainable bioenergy potential in 2050 to be between 38 and 102EJ per year maximum. This potential is far smaller than that found in South America and Africa.

If the US wants a substantial amount of bioenergy to be part of its future energy mix, it might have to look at imports from Africa and South America where the potential is many times greater. Europe, with its even smaller domestic land potential, acknowledges that imports from the South will have to play a critical role to ensure a more sustainable bioenergy supply and to meet its renewable energy targets.

On another note, the researchers do not hint at the biochar option. This technique consists of coupling carbon negative biofuel production to the production of charcoal which can be sequestered into soils to perform functions similar to biomass residues, but in a more efficient way. Moreover, biochar amended soils could act as a carbon sequestration method and yield carbon credits. Research into the technical and economic feasibility of biochar is ongoing (more here and references in that text).

References:
W. W. Wilhelma, Jane M. F. Johnsonb, Douglas L. Karlenc and David T. Lightle, "Corn Stover to Sustain Soil Organic Carbon Further Constrains Biomass Supply", Agronomy Journal, 99:1665-1667 (2007), Published online 6 November 2007, DOI: 10.2134/agronj2007.015

Edward M.W. Smeets, André P.C. Faaij, Iris M. Lewandowski and Wim C. Turkenburg, "A bottom-up assessment and review of global bio-energy potentials to 2050", Progress in Energy and Combustion Science, Volume 33, Issue 1, February 2007, Pages 56-106, doi:10.1016/j.pecs.2006.08.001

Biopact: IEA report: bioenergy can meet 20 to 50% of world's future energy demand - September 12, 2007

Biopact: Towards carbon-negative bioenergy: U.S. Senator introduces biochar legislation - October 07, 2007