By Stephen Thompson, Assistant Editor
stephenathompson@wdc.usda.gov

ellulosic biofuels offer promise for a sustainable future, but only if certain precautions are taken, according to speakers who addressed: “Biomass for Energy & Conservation: Can We Do Both?” It was one of dozens of panel talks that focused on a wide array of agricultural and rural issues at the 2010 USDA Agricultural Outlook Forum in February, held just outside Washington D.C.

Rattan Lal, professor of soil science at Ohio State University, called for restraint in using crop residues for generating fuels such as ethanol. Lal reminded the audience that ethanol produced from corn alone will be unable to meet future production targets, leaving “second generation” ethanol produced from cellulose to fill the gap.

Crop residues offer a tempting source of cellulose for this purpose, said Lal. However, he continued, these residues are extremely valuable for the maintenance of soil quality. Good crop production, he said, is dependent on returning most crop residues back to the soil, maintaining what he called “humus capital.”

Removing an excessive amount of crop residues can lead to loss of soil nutrients, erosion and the reduction of vital biological activity, such as beneficial microbes and earthworms, Lal said. Soil loss from erosion, he noted, is directly related to loss of mulch cover, while earthworm activity is vital for proper hydraulic conductivity in the soil. He told the audience that higher levels of soil organic carbon, maintained by retaining crop residues, are correlated with higher grain and soybean yields.

Lal said that up to 25 percent of total biomass could be harvested safely. However, he cautioned against the idea that cellulosic ethanol can be produced using low inputs on marginal soils. He maintained that replacing 10 percent of petroleum fuel requirements in the United States would require using 43 percent of current cropland. He suggested that mitigation of carbon dioxide emissions could be better accomplished by increasing fossil fuel efficiency, conserving and restoring forests and grasslands and sequestering carbon in soils.

Joe DiTomaso, an ecologist at the University of California-Davis, told participants that perennial grasses grown as cellulosic biofuels crops could pose a danger of escaping fields and becoming invasive. Such grasses include switchgrass, Miscanthus X. Giganticus, and giant reeds. Many of the characteristics that make them attractive as biofuel crops are also the characteristics of weeds, he said. Those traits include competitiveness, pest resistance and tolerance to drought, salinity and low fertility. Indeed, some of the species now proposed as cellulosic crops are already listed as noxious weeds in some localities, he said.

DiTomaso suggested that precautions be taken before a perennial grass crop is adopted. Those would include choosing crops that have been shown not to have high risks of invasiveness in the target area and taking active measures to minimize escape. For instance, he said, standard, non-sterile switchgrass is not suitable for cultivation in California because of a high escape risk. Cultivation areas must be chosen with caution as well, he said, and the conversion of natural habitats should be avoided.

Other suggestions included breeding crop varieties to minimize invasive traits; developing individual antidispersal, management and eradication protocols before planting; and establishing and funding an industry plan to quickly detect and eradicate escaped crops.

Douglas Karlen, of the Soil, Water, and Air Resources Research Unit of the USDA Agricultural Research Service in Ames, Iowa, repeated Lal’s assertion that crop residues are vital for maintaining soil quality. He said that an “industrial” approach must be avoided when contemplating feedstocks for biofuels in order to avoid causing environmental damage. Preserving landscape diversity allows the land to fulfill vital functions beyond growing crops, including sequestering carbon, providing wildlife habitat, recycling nutrients and protecting soil and water quality.

Karlen said that each farm should be approached as a system, and that addressing individual problems separately should be avoided. “There’s no single solution,” he told the audience. Instead, local landscape characteristics and other factors should be taken into account when choosing feedstock crops, and different conversion methods — such as thermochemical pyrolisis and traditional fermentation — should be evaluated in each instance.

According to Karlen, questions that should be answered when evaluating farming practices include: In all cases, he said, resource needs must be matched with appropriate conservation practices, and landscape plans must be constantly evaluated and modified as needed.




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