Researchers find trade-off between water quality

image: University of Illinois researchers including Giovani Preza-Fontes (pictured) assessed the nitrous oxide emission potential of agricultural practices designed to minimize nitrogen inputs to waterways .
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Credit: University of Illinois

URBANA, Ill. — With water quality guidelines requiring more farmers to act on nitrogen loss, cover crops and split nitrogen applications are becoming more common in the Midwest. But new research from the University of Illinois shows that these conservation practices may not provide environmental benefits across the board.

“As researchers, we tend to focus on one type of pollution at a time. Ours is one of the first studies to assess the nitrogen cycle more holistically. Conservation practices related to water quality have attracted a lot of attention lately, but it’s also important to know how they might affect emissions of nitrous oxide, a major greenhouse gas that contributes to the climate change,” says Giovani Preza-Fonteswho worked on the study as a doctoral student in the Department of Crop Sciences at U of I. Preza-Fontes is now a postdoctoral fellow at Purdue University.

When it comes to greenhouse gases, nitrous oxide is a doozy. With 298 times the potency of carbon dioxide, nitrous oxide is released when soil microbes metabolize nitrogen, an essential nutrient needed to grow corn. As soils warm up in spring and summer, microbes get to work on the nitrogen not taken up by crops, turning some of it into a potent greenhouse gas.

Some farmers have moved away from fall anhydrous application, distributing nitrogen between early spring and mid-season. Proponents say this split application is more precise, delivering the exact amount of nitrogen crops need at the time and leaving less in the soil to lose to drainage water. Similarly, farmers plant cover crops to absorb any excess nitrogen after the crops are harvested.

But how do these practices behave in terms of nitrous oxide emissions?

Preza-Fontes, with assistant professors Laura Christianson and Cameron Pittelkow (now at U.C. Davis), measured nitrous oxide emissions in corn fields over three seasons. They mimicked various conservation practices, including a pre-season application of nitrogen only; split nitrogen applications in the spring and at the V6 or V7 stage of corn; split nitrogen plus a cereal rye cover crop; or without added nitrogen.

They found that switching from pre-season to mid-season split nitrogen application did not influence nitrous oxide levels in any of the three years.

Instead, nitrous oxide spiked in plots with cover crops.

“During all three years, we observed larger daily peaks in nitrous oxide emissions with the combination of in-season split nitrogen application and cover crops, but these peaks did not translate by higher cumulative emissions, except for one year. I think this pattern indicates that there is potential for higher nitrous oxide emissions when we incorporate a cereal rye cover crop into our cropping systems,” says Preza-Fontes.

Why would cereal rye increase nitrous oxide emissions? Simply put, it can create the ideal conditions for the production of nitrous oxide. Dead plant matter is a massive plate for soil microbes, providing additional carbon for microbial activity. Plant residues also help create an oxygen-limiting environment, a key condition for the production of nitrous oxide.

Interestingly, the peaks appeared in the summer, not immediately after the end of the cover crop.

“Nothing happens on the ground, on the operations side, in July or August. The cover crop had been dead for months by then. So for future research, we’ll want to find management strategies to do in the spring to help reduce the spikes that come with the cover crop later in the summer,” says Christianson.

The study used a scorching glyphosate treatment to kill cover crops. The researchers say that a different termination method, schedule, or cover crop species could alter the potential for nitrous oxide emissions, but they don’t yet know how.

“We typically use the carbon-to-nitrogen (C to N) ratio as an indication of how fast the biomass will decompose. A grass like cereal rye has a higher C/N ratio, which means it will break down more slowly than a legume cover crop, which has a low C/N ratio,” says Preza-Fontes.

Regardless, the researchers point out that cover crops have been proven time and time again to improve water quality. And although cumulative emissions were greater in one year with cover crops, the practice of conservation still has value.

“There is no perfect practice, but that doesn’t mean we should give up on them all. We know that cover crops are good for water quality. The next steps would be to optimize cover crop species, management and tillage, so that we can continue to use cover crops for water and reduce peak nitrous oxide later in the year. summer,” says Christianson.

Preza-Fontes adds, “We should not rely on one practice to solve all environmental problems. I think knowing the limits and benefits of each practice will be the most appropriate way forward.

The study was done at Dudley Smith Farm. Christianson and his colleagues plan to host a field day in Taylorville on Feb. 10, where they will discuss ongoing research into on-farm conservation practices and water quality. register here.

The article “In-season split nitrogen application and cover cropping effects on nitrous oxide emissions in rainfed maize” is published in Agriculture, Ecosystems and Environment [DOI: 10.1016/j.agee.2021.107813]. Authors include Giovani Preza-Fontes, Laura Christianson, Kristin Greer, Rabin Bhattarai and Cameron Pittelkow. The research was supported by the Dudley Smith Initiative, the Illinois Nutrient Research and Education Council, the Foundation for Food and Agriculture Research, and the 4R Research Fund.

The Department of Crop Sciences is in the College of Agricultural, Consumer and Environmental Sciences to University of Illinois Urbana-Champaign.


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