The researchers found that 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 bio-fuel, pumping up the rate of nitrogen fertilization actually makes it more difficult to extract ethanol from corn leaves and stems.
This happens, they discovered, 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 team led by postdoctoral researcher Morgan Gallagher as part of her dissertation at Rice; The paper was published in online edition of the American Chemical Society's journal Environmental Science and Technology.
The findings are an important next step in building a sustainable bio-fuel economy. Plants benefit from some nitrogen from fertilizer to produce the bio-molecules they need to grow and function, said Carrie Masiello, an assistant professor of Earth science at Rice and Gallagher's adviser. 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 bio-fuel crop quality too," Masiello said.
While farmers have a clear incentive to maximize grain yields, the research shows a path to even greater benefits when corn residues are harvested for cellulosic ethanol production, she said.
The study, conducted at and in collaboration with the National Science Foundation's W.K. Kellogg Biological Station at Michigan State University (MSU), showed 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 said. At the same time, the researchers found only a modest increase in plant and stem cellulose, the basic component used to produce cellulosic ethanol.
“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,” Gallagher said.
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.
The researchers found that lignin yields from plant residue increased at nearly twice the rate as cellulose in response to nitrogen fertilization, and they said this implies “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," said 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 said.
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.
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