Nitrogen loss is also impacting the atmosphere
By KRISS NELSON
The leaching of nitrates has been on the forefront of environmental issues for several years, but recent data is showing the growing concern of what nitrogen fertilizer is doing to the atmosphere.
Iowa Learning Farms hosted a webinar last week called Wet Spots and Hot Spots: Understanding and Managing Nitrogen Losses from Hydric Soil Landscapes.
It was presented by Dr. Steven Hall, Iowa State University assistant professor of ecology, evolution and organismal biology.
Hall is leading a research group that studies the biological and geochemical processes that control the cycling of organic matter and nutrients across the plant-soil-water-atmosphere continuum.
One of the group’s focus areas are the interactions across the continuum in former prairie potholes. According to information from Iowa Learning Farms, these occasionally-flooded hydric soils in topographic depressions can contribute disproportionately to nitrogen losses at the landscape scale, suggesting the promise of management interventions that specifically target these features.
“We’re all really aware of the importance of nitrate loads from agricultural systems,” Hall said, “both in terms of the public health impacts from surface and ground water degradation, as well as when we get down to the Gulf of Mexico and the importance of nitrogen in terms of driving this dead zone in the Gulf.”
But he posed the question to the audience: “Are you at a sense of the relative importance of different aspects of Corn Belt ag systems and their impacts on climate change?”
Hall said the single largest cause of climate change from an agricultural system in the Corn Belt is nitrous oxide emissions from soil and water.
“I think the importance of nitrous oxide doesn’t receive as much attention as it deserves,” he said. “Nitrous oxide, as it turns out, is the third most important greenhouse gas on a global scale.”
In addition to a climate impact, Hall said nitrous oxide is the most important substance that’s depleting the stratospheric ozone.
“If we tally up the sources of nitrous oxide that come from agricultural soils, they are the single largest human drive source and within the United States, it turns out the Corn Belt is a major source of nitrous oxide emissions,” he said, adding the loss of nitrous oxide, also known as n2O, is high enough that, in many cases, it is likely to outweigh any benefits producers might get for enhancing carbon sequestration in agriculture.
“That is a really important message to get out there,” he said. “There’s huge advances we can make in terms of soil health and in terms of trying to build up organic matter in carbon, but I think it’s also really important to recognize wherever we are using fertilizer agricultural systems with nitrogen, these nitrous oxide emissions are tremendously important to consider in terms of the overall environmental impacts.”
Hall said there has been research conducted at ISU that documents the potential for significant gains of soil carbon/soil organic matter in well-managed corn that nitrous oxide is typically lost.
“That n2O loss either equals or outweighs carbon we might be gaining,” he said. “It’s really critical on how we might try to reduce these nitrous oxide emission in the context in improving soil health and sort of building resiliency in our Corn Belt systems.”
Hall said it is important to think about these two environmental problems of nitrogen: nitrate leaching and nitrous oxide emissions as being linked through crop nitrogen use efficiency.
“On balance, the more of our applied nitrogen that is getting taken up by the crop, the less that is sort of available for loss or as either nitrous oxide or other nitrogen gases or leaching losses of nitrates,” he said. “This holds both for considering fertilizer or manure we are applying and also in terms of the ambient soil nitrogen pools that are turning over and potentially being lost as n2O.”
Soil moisture also plays a role in both of those factors.
“We need rainfall to move nitrates through the soil system, out the tile system and to the streams,” he said. “Moisture is also very important in terms as to determining whether nitrogen is going to be lost as n2O or via some other form.”
Importance of soil drainage
A region that extends from Alberta and Saskatchewan in Canada down through Iowa is considered the Prairie Pothole Region (PPR) and was a result of one of the last glacier’s activity.
“As a consequence, the drainage and stream networks are fairly poorly developed and prior to European settlement we had a landscape that was dotted with all of these prairie pothole wetlands,” Hall said.
While some of this land has been put into the conservation reserve program (CRP) or has been left undisturbed, Hall said the majority of land in the PPR is used for crop production.
“Most of these are drained to some extent,” he said. “Some might have surface inlets, some of them don’t. But most of them are cropped, and as we are intuitively aware while driving around the landscape, especially this year, we get significant crop mortality in these depressions.”
Most of these areas tend to respond to large rainfall events by showing brief and/or long lasting ponding.
There is also a high level of yield suppressions around the pothole depressions.
“There is a significant landscape yield penalty, and the bottom line is, it’s an economic impact to growers,” he said.
In addition to those economic impacts, Hall said there are really key potential environmental impacts from these depression features in terms of nitrate loss and nitrous oxide emissions.
What are some different alternatives for addressing these potential environmental impacts of nitrate loads and nitrous oxide emissions?
“How might we think about changing this dynamic? Does it make sense to, year after year, plant corn and beans in places that are typically flooded out?” asked Hall. “Economical analysis is that these depressions are consistent money pits and there is a better scenario to mitigate some of these environmental impacts from these features.”
Some options that Hall believes might deserve greater attention include conservation tillage, CRP, flood-tolerant crops or more drainage intervention.
“There could be benefits from conservation tillage in terms of building soil structure and decreasing the duration of flooding in these features but promoting infiltration of moisture,” he said. “Some suggest the premise of conservation tillage, in terms of decreasing the severity of flooding in these depressions – hydrologic modeling – show we can significantly decrease the amount of pothole area that is flooded for more than two days.”
Targeting applications of CRP to those pothole depressions is another option.
“There are many people doing this already,” he said. “But there is certainly a lot of additional potential opportunities if we get more CRP funding on the federal level.”
Flood-tolerant crops are another option, especially for larger depressions where it might make logistical sense.
“It’s worthwhile to consider potential crops, such as bioenergy feed stocks – really productive flood-tolerant crops in these more poorly-drained areas – and if these crops can essentially increase nitrogen use efficiency, maybe draw down the moisture, we may be able to mitigate some of these environmental impacts,” he said.
Hall said producers in the PPR may want to consider further kinds of drainage intervention.
“It might make more sense from an environmental perspective to actually try to beef up these tile mains in some of these systems to enhance drainage; increase the productivity of the land,” he said. “If we better drain some of these depressional features, perhaps we can actually increase productivity while decreasing nitrogen losses. Both as nitrates and n2O.”
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