When corn stalks and stover are going to be used for ethanol production, using less nitrogen produces a better quality feedstock, according to research published in the online edition of the American Chemical Society journal, Environmental Science and Technology by scientists at Rice University.
While conventional wisdom encourages the use of more nitrogen fertilizer to boost grain production, excess nitrogen in the leaves and stacks speeds up the production of lignin, which has to be broken down to produce cellulosic ethanol.
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.
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, he said.