AMES, IOWA, US — Researchers from Iowa State University have shared modeling that could reduce emissions from growing soybeans by one-third while reducing yields.

In the analysis published in Nature Sustainability, a team led by Michael Castellano, agronomy professor and William T. Frankenberger Professor of Soil Science at Iowa State University, showed the reduction was possible by planting winter cover crops in the fall and soybeans earlier in the spring.

During a two-year rotation of corn and soybeans, about 40% of nitrous oxide emissions are in the soybean year, the analysis found.

The share of the potent greenhouse gas released during the soybean half of a crop rotation cycle is surprisingly high, given most soybeans fields aren’t treated with nitrogen, Castellano said.

“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,” he said. “Nearly half of our emissions in a typical cropping system come from soybeans, and we haven’t even been thinking about how to manage them.”

Castellano said his favorite part of this research is that farmers are more likely to implement the solutions right away because they’re practical and scalable.

It makes sense that most of the attention devoted to emissions from Corn Belt fields has been on optimizing nitrogen fertilizer usage, said Castellano, who also leads the Iowa Nitrogen Initiative, an ongoing research project to give Iowa farmers more precise data on ideal nitrogen rates.

Properly managing fertilizer use is the essential first step in reducing emissions in corn-soybean systems and has other environmental benefits, such as improving water quality.

But fertilizer isn’t the only source of nitrous oxide in farming. When microbes break down organic matter in soil, some of the nitrogen produced converts into a gas form. Without plants to use the nitrogen generated in decomposition, bare soil gives off higher amounts of nitrous oxide — especially in the spring, when warmth and moisture encourage microbial activity. That’s what causes most soybean emissions.

A strategy the researchers studied in their paper would drastically shorten the time farmland spends without living plants. Aerially sowing a winter cover crop of oats or rye into mature corn fields would cover soil with plants for the months between crops, and using an extended-growth soybean variety would allow earlier spring planting. The two-prong method would reduce soybean-year emissions by 33% and, with planting moved up about four weeks, increase yields by 16%, based on crop-system modeling.

Extended-growth soybeans are widely available, and farmers are increasingly pushing soybean planting earlier. But they usually prioritize corn in the spring because corn yields are more affected by a late start than soybean yields. It helps that the US Department of Agriculture in 2023 moved up by about a week the earliest planting date for soybeans covered by federal crop insurance. Soybeans in most of Iowa are covered if planted on or after April 15, while corn across Iowa and soybeans in southern Iowa are covered on or after April 10.

In the future, federal officials should consider the environmental benefits of setting earlier planting dates for insurance coverage, Castellano said.

“Our work suggests earlier planting has some value that should be incorporated when making those decisions,” he said.

The United Soybean Board provided the primary funding for the research and is supporting a three-year follow-up study to field test the emissions and yield impact of combining a corn-following cover crop and earlier soybean planting, Castellano said. Results at experimental plots in Iowa, Illinois, Minnesota and Kentucky have been promising.