Less fertilizer is more for biofuel crops

RICE (US) — Liberal use of nitrogen fertilizer to maximize yield from corn grown for ethanol is not only bad for biofuel production, it’s bad for the environment as well.

The findings have implications for an industry that has grown dramatically in recent years to satisfy the country’s need for energy while trying to cut its reliance on fossil fuels.

Details are reported online in the journal Environmental Science and Technology.

Corn grain, one source of ethanol, and the stalks and leaves, the source of cellulosic ethanol, respond differently to nitrogen fertilization.

Liberal use of nitrogen fertilizer to maximize grain yields from corn crops results in only marginally more usable cellulose from leaves and stems. And when the grain is used for food and the cellulose is processed for biofuel, pumping up the rate of nitrogen fertilization actually makes it more difficult to extract ethanol from corn leaves and stems.

This happens because surplus nitrogen fertilizer speeds up the biochemical pathway that produces lignin, a molecule that must be removed before cellulosic ethanol can be produced from corn stems and leaves.

The findings are an important next step in building a sustainable biofuel economy, says Carrie Masiello, assistant professor of earth science at Rice University. Plants benefit from some nitrogen from fertilizer to produce the biomolecules they need to grow and function, but for many crops, a little is enough.

“We already know too much fertilizer is bad for the environment. Now we’ve shown that it’s bad for biofuel crop quality too,” Masiello says.

While farmers have a clear incentive to maximize grain yields, the esearch shows a path to even greater benefits when corn residues are harvested for cellulosic ethanol production, she says.

The study shows that although feeding the plant more fertilizer increases the grain’s cellulose content, grain yield quickly hits a plateau.

“The kilograms of grain you get per hectare goes up pretty fast and peaks,” Masiello says. At the same time, only a modest increase in plant and stem cellulose, the basic component used to produce cellulosic ethanol was recorded.

“The implicit assumption has always been that the response of plant cellulose to fertilizer is going to be the same as the grain response, but we’ve showed this assumption may not always hold, at least for corn,” says postdoctoral researcher Morgan Gallagher who led the research as part of her dissertation.

Nitrogen fertilization encourages production of lignin within the plant, and without lignin, stalks won’t stand. Lignin production comes at the expense of useful cellulose production.

Lignin yields from plant residue increased at nearly twice the rate as cellulose in response to nitrogen fertilization, implying “that residue feedstock quality declines as more nitrogen fertilizer is applied.”

Lignin breaks down slowly via bacterial enzymes, and it is expensive to remove by chemical or mechanical processes that create a bottleneck in cellulosic ethanol production.

“The ideal cellulosic ethanol crop has no lignin—except you can’t have a plant without it, because it would fall over. Plants need some lignin to maintain structure,” says co-author Bill Hockaday, a former Rice postdoctoral researcher and now an assistant professor at Baylor University. “What we want is a low lignin-to-cellulose ratio.”

Reducing fertilizer to the bare-bones minimum serves that purpose. “Morgan showed that if you look at kilograms of cellulose per hectare, yields don’t increase at the same rate for the grain and the leaves and stems. There’s really only a small amount of fertilizer needed if you’re cropping strictly for cellulose,” Masiello says.

Overfertilization also increases the decomposability of corn residue plowed back into the fields. This implies that soil carbon storage becomes less efficient—another minus for the environment because storing additional carbon in soil can reduce the atmospheric concentration of carbon dioxide and help crops access soil water.

Issues associated with the runoff of nitrogen from fertilizer into streams and leaching into groundwater are common knowledge, Masiello says, noting the link between nitrogen fertilizer use in the Mississippi Valley and a “dead zone”—defined as a lack of life-supporting oxygen—in the Gulf of Mexico.

Nitrate runoff and leaching into drinking water supplies has also been linked to a number of health problems.

Gallagher notes improving the yield of feedstock for cellulosic ethanol leaves more corn for food.  “There’s a billion people who are malnourished, so it’s ethically questionable to use corn grain for fuel rather than food.”

The researchers hope their methods can be transferred to other crops grown for ethanol.

Researchers at Michigan State University contributed to the research, that was supported by the National Science Foundation.

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