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Does More Fertilizer Make For More Ethanol?

tractor spraying fertilizer on farm field
Overfertilization of corn may impair cellulosic ethanol production.
Soil Science

Dump more fertilizer on a field of growing corn, and what do you get? More corn? Not necessarily. And if that corn is to be used not just for food, but for cellulosic ethanol? Again, the results may not be what you’d expect. A new study by researchers from Rice University indicates that overfertilization of corn, besides being harmful to the environment, may actually impair cellulosic ethanol production.

The study, published in the online edition of the American Chemical Society’s journal Environmental Science and Technology, examined the effect of nitrogen fertilizer on yields of both corn grain, and corn stocks and leaves. Of course, corn grain is a common food staple while the stocks and leaves are a primary feedstock in cellulosic ethanol production. (Corn grain can also be used to produce ethanol, but doing so takes us further into the food versus fuel debate.) Of note is that each part of the plant responds differently to high nitrogen levels.

Morgan Gallagher, the lead researcher on the study, comments, “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.”

Here’s why. Plants need nitrogen to grow. While nitrogen is abundant in the atmosphere, plants can not readily use it in this form. Natural processes, like lightning and the enzymatic action of some types of bacteria, will fix atmospheric nitrogen (N2) into organic nitrogen compounds plants can use. But today, most large-scale farming operations supplement the process with nitrogen-rich fertilizers.

What happens when nitrogen levels rise above a certain point? The study indicates that grain production in corn plateaus; above this level, more nitrogen does not produce more grain. But high levels of nitrogen do cause the plants to produce more lignin. This complex molecule forms part of the cell wall and is essential to plant health. Without it, the maturing corn would soon collapse under its own weight.

Lignin, however, impedes the production of cellulosic ethanol; lignin must be removed before the cellulose in stems and leaves can be converted to ethanol. Thus, high nitrogen levels in fertilizer actually inhibit cellulosic ethanol production.

It should be noted as well that nitrate runoff from over-fertilized farm fields can impact drinking water and has been linked to oxygen-depleted “dead zones” like that in the Gulf of Mexico and elsewhere. Carrie Masiello, Gallagher’s adviser on the study, explains, “We already know too much fertilizer is bad for the environment. Now we’ve shown that it’s bad for biofuel crop quality too.”

In at least one sense, this is good news for farmers; there’s no need to over-fertilize crops. Doing so would be counter-productive and expensive. It does, however, require more careful planning, measurement, and analysis. Further, it suggests that with proper management, there’s no reason the biofuel and food industries can not peacefully co-exist and even benefit one another.

Read Biofuels and Bioenergy: Processes and Technologies from Amazon.

Jules Smith is the principal of LightningStrike Studios, a professional writing firm specializing in renewable energy and information technology. Visit www.lightningstrikestudios.com to learn more.

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