Houston, Texas – Using too much fertilizer on corn will result in less ethanol when it’s processed, according to a study by researchers at Rice University in Texas. The research has implications for an industry that has grown drastically in recent years with increased focus on the new fuel.
Corn grain, one source of ethanol, and the stalks and leaves, the source of cellulosic ethanol, respond differently to nitrogen fertilization. The researchers found that liberal use of nitrogen fertilizer to maximize the grain yields from corn crops results in only marginally more usable cellulose from leaves and stems. 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 the leaves and stems. This is because surplus nitrogen fertilizer speeds up the biochemical pathway that produces lignin, a molecule that must be removed before the stems and leaves can be processed into cellulosic ethanol.
Plants benefit from some fertilizer nitrogen to produce the bio-molecules they need to grow and function, said Carrie Masiello, an assistant professor of earth science at Rice, 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 said.
The study, conducted at and in collaboration with the National Science Foundation’s W.K. Kellogg Biological Station at Michigan State University, showed that although feeding the plant more fertilizer increases the grain’s cellulose content, grain yield quickly hits a plateau. 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 kilograms of grain you get per hectare goes up pretty fast and peaks,” Masiello said. “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.”
Nitrogen fertilization encourages the production of lignin within the plant. Lignin is necessary to keep the plant upright, but it comes at the expense of useful cellulose production, since it breaks down slowly via bacterial enzymes and is expensive to remove by chemical or mechanical processes. The researchers found that 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. Reducing fertilizer to a minimum results in a low lignin-to-cellulose ratio that is enough that the plant stands upright. “There’s really only a small amount of fertilizer needed if you’re cropping strictly for cellulose,” Masiello said.