Nitrogen Burn-Out

It’s time to put one more crop farming myth out to pasture: it turns out applying lots of nitrogen fertilizer in fact does not reduce global warming. For decades, Midwestern farmers have been advised to pour on the nitrogen. The idea is to feed those corn plants a critical nutrient, and thus maximize yields. It’s created a bit of a treadmill effect: as better hybrids and other advances increase yields, farmers have been advised to apply higher doses of fertilizer in an effort to replace the increasing amounts of nitrogen being removed from the soil by all that nutrient-hungry super-corn.

In recent years, another argument for applying lots of nitrogen has been that since it increases soil fertility, it helps trap carbon, thus reducing greenhouse gas emissions. But a study out of the world’s oldest experimental field under continuous corn finds that excessive nitrogen fertilizer actually accelerates decomposition of crop residue such as corn stalks by sending soil microbes on a kind of manic feeding frenzy. In fact, research at the famous Morrow Plots at the University of Illinois found that heavy doses of nitrogen not only burn through all the crop residue, but also start working on the organic carbon in the soil itself, reducing the carbon by an average of 4.9 tons per acre. Over a 50-year period, this reduced yields in continuous corn by 20 percent, and released a lot of carbon dioxide into the atmosphere. These results are confirmed by field studies.

This research, which was published in the Nov./Dec. 2007 issue of the Journal of Environmental Quality, is significant for a lot of reasons, not the least of which is that the results come out of a highly-respected experimental field that’s been around for 100 years. Since synthetic nitrogen fertilizer was first applied to the Morrow Plots in 1955, researchers have been able to compare the impact on soil carbon and organic matter content both pre-nitrogen and post-nitrogen.

The timing of this research paper is key as well: there’s a lot of talk these days about using corn stover as a source of cellulosic biofuel. This research shows we need to do two things: 1) apply nitrogen fertilizer in a much more precise manner that’s appropriate for the local conditions found on individual fields and 2) stop immediately any talk of using crop residue to fuel our cars—removing that residue is bad for the soil, bad for our water and bad for the climate.