In a surprise, study finds substantial emissions in soybean growing season

Throughout his nearly 20 years of farming full-time, Corey Goodhue has grown a range of crops on his Central Iowa farm near Carlisle — everything from peas to sweet corn, wine grapes to alfalfa and even some landscape trees.

But Goodhue, who is starting his second year on the board of directors for the Iowa Soybean Association, said he didn’t anticipate the findings from a recent Iowa State University study. The study, published Oct. 7, found that 40 percent of nitrous oxide emissions — a greenhouse gas — stem from the soybean growing season.

“It was a surprising result because we don’t apply nitrogen to soybeans,” Goodhue said.

ISU’s study

Tomas Della Chiesa, a postdoctoral research associate for ISU, said he also wasn’t anticipating that the soybean season would account for so much of the annual emissions when he started his research about two years ago.

“This number is surprisingly higher than what we expected,” Della Chiesa said.

Della Chiesa said nitrous oxide — or N2O — is a gas emitted into the atmosphere that causes two major environmental impacts: It is the primary compound responsible for the depletion of the ozone layer and a potent greenhouse gas with 300 times the warming potential of carbon dioxide.

“N2O is the third greenhouse gas in the ranking of the global warming caused behind CO2 and methane (CH4),” Della Chiesa said in an email.

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The surprising find comes not because of soybeans themselves, but because of the normal two-year rotation of growing corn and then soybeans in the same field.

Della Chiesa and Mike Castellano, an ISU professor of agronomy, found that over the typical two-year rotation, 40 percent of the annual nitrous oxide emissions are emitted during the soybean-growing season. This is because when soybeans are grown in the same soil the year after corn was harvested, a lot of residue is left — including stover, the stalks, leaves or husks left after a corn harvest.

Matt Herman, the chief officer for demand and advocacy for the Iowa Soybean Association, said the soil starts to convert corn stover into nitrate and into N2O gases after a harvest. “And that conversion process happens in the spring and the summer when those soybeans are growing the next year,” Herman said.

Herman said much of these N2O gases picked up during the soybean season are “left over” from the corn season.

“That’s why it was so surprising to us,” said Herman, who had been working with Della Chiesa and Castellano on the research. “Because it’s a really low input crop, but where are all these emissions coming from?”

According to data collected by the U.S. Department of Agriculture, 2.6 million bushels of corn and 638,080 bushels of soybeans were produced in 2024 in Iowa.

Della Chiesa said that 30 percent of greenhouse gas emissions in Iowa come from agriculture. He said this is partly because other industries — like transportation — have found ways to mitigate emissions, while agriculture hasn’t as much.

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Castellano said the agriculture industry and researchers alike “really need” to be forward-thinking about opportunities to reduce emissions. He said an example of this includes farmers investing in cover crops.

“The big challenge is the fact that we're not growing things for nine months of the year, and we have these really rich, organic-matter soils … they're going to produce nitrate whether the crops are growing or not,” Castellano said.

Another way farmers can reduce the amount of nitrous oxide that is emitted during the soybean season is by lengthening the time farmers grow crops throughout the year.

“By moving that planting date up, growing a longer, mature variety which yields higher, means it takes up more nitrogen from the soil, and planting that cover crop, we made the substantial emissions reductions,” Castellano said.

Making the growing season longer, Della Chiesa said, is a “win-win” for both farmers and the environment.

“There is not only a reward on the environmental aspect by advancing a planting date, but also from the farmers or the producer’ perspective, too, because we are seeing that advancing the planting date has implied a higher yield,” Della Chiesa said.

Della Chiesa said the current average planting date for soybeans in Iowa is at the end of May. Going forward, he said an ideal soybean planting date would be mid- to late April to improve yields and curb N2O emissions — but it is dependent on when the ground warms up after winter.

Castellano said there are tactics farmers can use to plant earlier in the season.

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He said that improving field drainage is one of the biggest practices a farmer can do to plant soybeans earlier. Castellano said that residue management — or making sure the fields are cleaned up after harvest — is another way farmers could plant earlier.

However, Castellano said many farmers have been planting their crops earlier in the season even before the study was published.

“Farmers are naturally planting earlier for a couple of reasons: One is that the yield is higher when you when you plant earlier,” Castellano said. “Two, springs are getting warmer, so there's an increased opportunity there to plant early. And third, I think most farmers understand that the optimum planting date for corn and soybeans is the same. Soybeans tends to go on the back burner, though, because there's less risk with soybean production than there is corn.”

Going forward, both researchers said their next step is studying corn emissions and if planting corn earlier in the season could reduce the nitrous oxide emissions rate also.

Castellano also said he and Della Chiesa just wrapped up their first year of research on a study supported by the United Soybean Board. The multiyear study is in partnership between ISU, the University of Illinois, the University of Kentucky and the University of Minnesota.

For the study, “we are putting pieces of the puzzle together to see how we can get the biggest emissions reductions,” Castellano said. “For example, in Iowa, we're using cover crops and strip tillage and early planting, because we think putting all three of those things together will lead to the greatest emissions reductions.”

But with Minnesota, for example, it's too cold to use cover crops, he said.

Minnesota “just doesn’t have enough growing degree days to get good cover crop growth,” Castellano said. “So what they're trying to do there is using no-till in combination with early planting. And they're thinking about some ways they can potentially warm up that soil earlier in the spring, while maintaining no-till you get those soybeans to grow earlier.“

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Investing in cover crops

Goodhue has been planting cover crops on his farm for over a decade.

He was using practices like no-till, but he said it wasn’t enough on its own to prevent topsoil loss. “We really needed to find another tool to add to our toolbox for those fields,” Goodhue said.

Planting cover crops ended up being the tool he needed. He initially started planting cover crops in 2012.

In 2023, Iowa farmers planted about 3.8 million acres of cover crops. Cover crop practices have been increasing throughout Iowa over the past several years, partly due to increased federal and state incentive programs — like the USDA’s Environmental Quality Incentives Program, which works to provide financial aid and technical assistance to agricultural producers.

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But, Herman said, these financial assistance and incentive programs, like Iowa’s cover crops cost-share program, are often not enough to entice farmers.

Iowa’s cover crops cost-share program, launched in May 2022, covered about $25 per acre of planting for farmers. As of May, Iowa Secretary of Agriculture Mike Naig increased the price to $30 per acre for eligible farmers.

However, Herman said there are other costs of planting cover crops — such as wear and tear on equipment and fuel — that aren’t covered by that funding.

“Iowa’s program is very helpful, and a lot of farmers use it, but on its own it's not enough,” he said.

Herman said farmers and agricultural workers should learn ways to “stack up” multiple incentive programs to make them more affordable.

Olivia Cohen covers energy and environment for The Gazette and is a corps member with Report for America, a national service program that places journalists in local newsrooms to report on under-covered issues.

Comments: olivia.cohen@thegazette.com