Cover Crops in Nitrogen’s Circle of Life

Feb 19, 2018

A circle of life–and nitrogen–is playing out in farms across the United States. And researchers are trying to get the timing right.

Some cover crops, such as hairy vetch or cereal rye, are not grown to be eaten. Instead, they capture nutrients, including nitrogen, from previous crops, the air, and the soil. When cover crops decompose, these nutrients are released. Cash crops, such as corn or soybean, planted afterward can use these nutrients to grow and thrive.

But cash crops need different amounts of nutrients at different stages of growth. A new study assesses how quickly nutrients are released from two different cover crops. The goal, according to study co-author Rachel Cook, is to time nutrient release from cover crops to better match the nutrient needs of specific cash crops.

“It's like trying to time a meal to come out of the oven exactly when all the hungry dinner guests arrive,” says Cook, currently a researcher at North Carolina State University.

The researchers focused on nitrogen because it “is typically the most limiting nutrient in crop production, but has the most potential for environmental impact from losses.” The two cover crops, hairy vetch and cereal rye, are two of the most commonly planted cover crops in the Midwest.

They found that hairy vetch and cereal rye had significantly different nitrogen release dynamics.

“We now better understand the rate and quantity of nitrogen release from two of the more popular cover crops currently in use,” says Cook. “This information can help farmers estimate how much nitrogen they might expect to get from their cover crop and when it will be available.”

The study showed that hairy vetch released more nitrogen overall compared to cereal rye. Nitrogen release was also quicker from hairy vetch plants whose growth had been halted.

“Hairy vetch releases almost all available nitrogen in the first four weeks after it's terminated,” says Cook. That’s before the major time of nitrogen uptake by corn, which is around week eight after planting. “So, terminating hairy vetch too early could cause losses of nitrogen before the corn crop can get to it.”

Cereal rye, on the other hand, released nitrogen slowly over multiple weeks. “This would be beneficial before a cash crop with low nitrogen needs,” says Cook.

The study was carried out in field test sites at the Agricultural Research Center at Carbondale, Illinois. Study plots were planted with either cereal rye or hairy vetch. After terminating the cover crops with herbicide, researchers planted soybean or corn, respectively.

The researchers measured the growth of the two cover crops, how quickly they decomposed once terminated, and the ensuing quantity and rate of nitrogen they released.

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Video: Why Your Food Future Could be Trapped in a Seed Morgue

In a world of PowerPoint overload, Rex Bernardo stands out. No bullet points. No charts. No jargon. Just stories and photographs. At this year’s National Association for Plant Breeding conference on the Big Island of Hawaii, he stood before a room of peers — all experts in the science of seeds — and did something radical: he showed them images. He told them stories. And he asked them to remember not what they saw, but how they felt.

Bernardo, recipient of the 2025 Lifetime Achievement Award, has spent his career searching for the genetic treasures tucked inside what plant breeders call exotic germplasm — ancient, often wild genetic lines that hold secrets to resilience, taste, and traits we've forgotten to value.

But Bernardo didn’t always think this way.

“I worked in private industry for nearly a decade,” he recalls. “I remember one breeder saying, ‘We’re making new hybrids, but they’re basically the same genetics.’ That stuck with me. Where is the new diversity going to come from?”

For Bernardo, part of the answer lies in the world’s gene banks — vast vaults of seed samples collected from every corner of the globe. Yet, he says, many of these vaults have quietly become “seed morgues.” “Something goes in, but it never comes out,” he explains. “We need to start treating these collections like living investments, not museums of dead potential.”

That potential — and the barriers to unlocking it — are deeply personal for Bernardo. He’s wrestled with international policies that prevent access to valuable lines (like North Korean corn) and with the slow, painstaking science of transferring useful traits from wild relatives into elite lines that farmers can actually grow. Sometimes it works. Sometimes it doesn’t. But he’s convinced that success starts not in the lab, but in the way we communicate.

“The fact sheet model isn’t cutting it anymore,” he says. “We hand out a paper about a new variety and think that’s enough. But stories? Plants you can see and touch? That’s what stays with people.”

Bernardo practices what he preaches. At the University of Minnesota, he helped launch a student-led breeding program that’s working to adapt leafy African vegetables for the Twin Cities’ African diaspora. The goal? Culturally relevant crops that mature in Minnesota’s shorter growing season — and can be regrown year after year.

“That’s real impact,” he says. “Helping people grow food that’s meaningful to them, not just what's commercially viable.”

He’s also brewed plant breeding into something more relatable — literally. Coffee and beer have become unexpected tools in his mission to make science accessible. His undergraduate course on coffee, for instance, connects the dots between genetics, geography, and culture. “Everyone drinks coffee,” he says. “It’s a conversation starter. It’s a gateway into plant science.”