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August 24, 2022
by Lauren Quinn,
University of Illinois at Urbana-Champaign
Nitrous oxide emissions from Corn
Belt soils spike when soils freeze and thaw.
Credit: Unsplash/CC0 Public Domain
Nitrous oxide may be much less abundant in
the atmosphere than carbon dioxide, but as a greenhouse gas, it's a
doozy. With a potency 300 times greater than CO2, nitrous
oxide's warming potential, especially via agriculture, demands
attention.
University of Illinois and University of
Minnesota researchers are answering the call. In a new study, they
document an overlooked but crucial timeframe for
nitrous oxide (N2O) emissions in U.S. Midwest
agricultural systems: the non-growing season.
"Nitrous oxide emissions from
agricultural soils have mostly been studied during the growing
season. Previous research shows non-growing season N2O
emissions can contribute up to 70–90% of annual emissions in some
years, but it's not clear how accurate that range is for the Midwest
or what processes and management practices contribute to those
emissions in the fall and winter," says Yufeng Yang, the study's lead
author and doctoral student at U of M.
Yang and his co-authors used a computer
simulation model known as ecosys to determine the hotspots and "hot
moments" for N2O emissions across the Midwest.
Specifically, they teased out the climate and environmental factors
contributing to N2O emissions on a county-by-county basis
during non-growing seasons between 2001 and 2020. They also looked at
the effects of
fertilizer application timing and nitrification inhibitors.
"This validation study demonstrates the
ecosys model can realistically simulate N2O emissions from
agricultural soils in the non-growing season. It means we now have a
robust way to quantify the contributions of environmental variables
and nitrogen application timing to this important
greenhouse gas," says study co-author Kaiyu Guan, associate
professor in the Department of Natural Resources and Environmental
Sciences and founding director of the Agroecosystem Sustainability
Center at U of I.
First, the researchers found the
non-growing season in the Midwest accounted for a wide range of annual
N2O emissions: 6 to 60%. The variation could be traced back
to differences at the county level, with emission levels diverging for
counties in the southeastern and northwestern extremes of the region.
For context, soil N2O
emissions are the result of microbial processes converting nitrogen
from one form to another. Environmental conditions, such as the amount
of moisture and oxygen in the soil, soil temperature, or the amount of
snowpack on the soil surface, affect how much and how quickly microbes
can metabolize nitrogen, as well as the ability of gaseous nitrogen
products to be released into the atmosphere.
The ecosys model detected these
environmental drivers across the region, highlighting greater
emissions in counties with more than 12 inches of non-growing season
precipitation. But the researchers looked for even more detail to
explain the pattern.
"More intensive freezing caused by
decreased air temperature was the dominant driver leading to increased
non-growing season N2O emissions in the southeastern
Midwest. In the northwest, increased precipitation and increased air
temperature during thawing cycles were the key drivers enhancing
non-growing season N2O production," Yang says.
The long-term outlook for these regional
differences may shift under a changing climate, however. Yang
simulated future climate scenarios and found less freezing and
thawing, potentially dampening the spikes that currently occur under
these conditions.
The team also found the effects of
nitrogen fertilizer application timing also varied by county.
Generally, emissions were greater under fall application than spring
application.
"Results suggest that shifting fall
application to spring application and applying nitrification
inhibitors at either time point can greatly reduce annual N2O
emissions at the regional scale, and can reduce nitrogen leaching as
well," says study co-author Ziyi Li, doctoral researcher studying
under Guan at U of I.
But that effect wasn't universal. Fields
in the west of the study area saw fewer emissions with fall
application.
"Scientists always suggest switching to
spring fertilizer application, but it's not a black and white story.
Our model enables farmers to receive targeted recommendations specific
to their fields," says Zhenong Jin, corresponding author, project
leader, and assistant professor in the Digital Agriculture Group at U
of M.
The researchers say the model could be
used to evaluate the effects of additional management strategies, such
as cover cropping and no-till, on N2O emissions.
"The bottom line is we now have a highly
accurate method for estimating N2O emissions at the county
scale in the Corn Belt. We have underestimated the non-growing-season,
but it turns out to be a pretty significant portion of annual N2O
emissions," Jin says.
The study is published in Agricultural
and Forest Meteorology.
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