Researchers find tradeoff between water quality and emissions on the
farm

Date:
January 27, 2022
Source:
University of Illinois College of Agricultural, Consumer and
Environmental Sciences
Summary:
With water quality guidelines compelling more farmers to act on
nitrogen loss, cover crops and split nitrogen applications are
becoming more common in the Midwest. But new research shows these
conservation practices may not provide environmental benefits
across the board.



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


==========================================================================
"As researchers, we tend to look at one type of pollution at a
time. Ours is one of the first studies to evaluate the nitrogen cycle
more holistically.

Conservation practices relating to water quality have gained a lot of
attention lately, but it's also important to know how they might affect emissions of nitrous oxide, an important greenhouse gas contributing to
climate change," says Giovani Preza-Fontes, who worked on the study
as a doctoral student in the Department of Crop Sciences at U of
I. Preza-Fontes is now a postdoctoral researcher at Purdue University.

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

Some farmers have moved away from the fall anhydrous application,
splitting nitrogen between early spring and mid-season. Proponents say
this split application is more precise, delivering the exact amount of
nitrogen crops need in the moment and leaving less in the soil to be
lost in drainage water.

Similarly, farmers plant cover crops to take up any excess nitrogen
after crops are harvested.

But how do these practices perform in terms of nitrous oxide emissions? Preza-Fontes, along with assistant professors Laura Christianson and
Cameron Pittelkow (now at UC Davis), measured nitrous oxide emissions
in corn fields across three seasons. They mimicked various conservation practices, including a pre-season nitrogen application only; split
nitrogen applications in spring and at the V6 or V7 corn growth stage;
split nitrogen plus a cereal rye cover crop; or no added nitrogen.



==========================================================================
They found shifting from pre-season to in-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.

"For all three years, we saw greater daily spikes in nitrous oxide
emissions with the combination of in-season split nitrogen application and cover crops, but those spikes did not translate into greater cumulative emissions, except for one year. I think this pattern indicates there is
a potential for greater nitrous oxide emissions when we incorporate a
cereal rye cover crop into our cropping systems," Preza-Fontes says.

Why would cereal rye increase nitrous oxide emissions? Simply put, it
can create the perfect conditions for nitrous oxide production. The dead
plant matter is a massive dinnerplate for soil microbes, providing extra
carbon for microbial activity. The plant residue also helps to create an oxygen-limiting environment, a key condition for nitrous oxide production.

Interestingly, the spikes came in the summer, not immediately after
cover crop termination.



========================================================================== "Nothing is going on in the field, operations-wise, in July or August. The cover crop has been dead for months at that point. So, for future
research, we'll want to figure out management strategies to do in the
spring to help reduce the spikes that come with the cover crop later in
the summer," Christianson says.

The study used a burndown glyphosate treatment to kill the cover
crops. The researchers say a different termination method, timing, or
cover crop species could change the potential for nitrous oxide emissions,
but they don't yet know how.

"We usually use the carbon-to-nitrogen (C to N) ratio as an indication of
how fast biomass will decompose. A grass such as cereal rye has a higher
C to N ratio, which means they will decompose slower than a legume cover
crop, which has a low C to N ratio," Preza-Fontes says.

Either way, the researchers emphasize cover crops have been proven many
times over to benefit water quality. And although cumulative emissions
were greater in one year with cover crops, the conservation practice
still has value.

"There's no perfect practice, but that doesn't mean we should abandon
all of them. We know cover crops are good for water quality. Next steps
would be to optimize the cover crop species, management, and tillage,
so that we can continue to use cover crops for water and reduce the
nitrous oxide spike later in the summer," Christianson says.

Preza-Fontes adds, "We shouldn't rely on only one practice to solve every environmental problem. I think knowing the limitations and advantages
of each practice will be the most appropriate way of moving forward."
The study was done at the Dudley Smith Farm. Christianson and her
colleagues plan to host a field day in Taylorville on Feb. 10, where
they will discuss continuing research on farm conservation practices
and water quality.

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 at the University of Illinois Urbana-Champaign.

========================================================================== Story Source: Materials provided by University_of_Illinois_College_of_Agricultural,_Consumer and_Environmental_Sciences. Original written by Lauren Quinn. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Giovani Preza-Fontes, Laura E. Christianson, Kristin Greer, Rabin
Bhattarai, Cameron M. Pittelkow. In-season split nitrogen
application and cover cropping effects on nitrous oxide emissions
in rainfed maize.

Agriculture, Ecosystems & Environment, 2022; 326: 107813 DOI:
10.1016/ j.agee.2021.107813 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220127172632.htm

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