Soybeans Have Surprisingly High Impact on Nitrous Oxide Emissions

According to a study sponsored by Iowa State University, soybean crops have surprisingly high nitrous oxide emission levels. They normally are not treated with soil. However, that does not mean natural processes in the soil stop.

Soybeans and corn are two common crops grown in Midwestern states like Iowa. Very often, they are rotated: Farmers will plant soybeans one year, and then plant corn in the same field the next year. The study says 40% of nitrous oxide emissions happen during the soybean year.

“We’ve just been assuming that legume crops like soybeans don’t have a big emissions footprint because they don’t usually receive fertilizer. But the natural processes in soil that produce nitrous oxide don’t stop just because you don’t apply fertilizer,” said Iowa State University agronomy professor Michael Castellano.

Nitrous oxide emissions are unavoidable in any agricultural operation that involves growing crops. These emissions can be reduced through appropriate fertilizer management, which also has benefits for water quality. However, they can’t be eliminated entirely due to microbial activity that doesn’t necessarily have to do with fertilizer. Microbes typically break down waste plant matter left over from previous years’ crops, for instance.

Most of the nitrous oxide emissions occur in the spring, which could indicate a surge in microbial activity that begins before the seeds are even planted. The study suggested that some of it could be mitigated by planting a winter crop and choosing an extended-growth soybean variety, which helps absorb nitrogen-based compounds that could turn into nitrous oxide.

One challenge: Farmers prefer to get corn planted first because it has a slightly longer growing season. Corn planting typically begins around April 10, and soybean planting typically begins around April 15. The research team thought some of this could be mitigated through adjustments in insurance policies to allow for earlier soybean planting.

The versatility of soybeans also presents a challenge because its many uses can drive up demand. Farmers often incorporate it in livestock feed. Margarine, salad dressing, and mayonnaise can be made using soybean oil. Of course soybeans are used in well-known vegetarian and vegan foods like tofu and “meatless meats” like imitation bacon, hamburger, and hot dogs. Consumer products made with materials like rubbers, fibers, plastics, coatings, solvents, lubricants, and adhesives might use some form of soy derivatives. So it’s probably not going to go away as a crop.

However, awareness of agriculture as a minor contributor to greenhouse gas emissions has been growing. Methane emission in cattle farms is merely one of the better-known examples. However, because so much land is used for producing food and demand for food is not going anywhere anytime soon, there aren’t many ways to combat it.

Nitrous oxide emission levels in soybean crops were surprisingly high, according to Iowa State University. Naturally, awareness is just the first step in combating the problem. It may be possible to mitigate the problem using methods that farmers can execute with a few adjustments.