Though carbon emissions get a lot of airtime in the media, at North Carolina State University (NCSU), a group of researchers is focusing on nitrous oxide. Increasingly, there’s a consensus that N2O is the most important greenhouse gas (GHG) for agriculture to address. Although nitrous oxide accounts for only 5 percent of GHGs overall, the nitrous oxide molecules stay in the atmosphere for an average of 114 years before being removed by a sink or destroyed through chemical reactions. The impact of 1 pound of N2O on warming the atmosphere is almost 300 times that of 1 pound of carbon dioxide. In the United States, agriculture accounted for 75 percent of nitrous oxide emissions in 2015, according to the U.S. Environmental Protection Agency. Emissions data coming from the southeastern United States is different from data coming out of other parts of the country. Researchers have been able to recognize this difference in recent years, thanks to a new measurement system designed by Professor Wayne Robarge, a soil scientist at NCSU’s College of Agriculture and Life Science.
“What happens for us is we’re emitting almost none and then we get a big rainstorm that really soaks the soil profile and we’ll have a brief burst [of N2O emissions] that’s pretty big and then dies off after a couple days,” said Chris Reberg-Horton, associate professor and organic cropping specialist at NCSU. “Identifying that it’s episodic and figuring out how to measure that has really been a big deal.”
Reberg-Horton said the data raises the possibility that nitrous oxide emissions in organic agriculture and conventional agriculture are driven by different things. The timing of when the greenhouse gas comes out of the organic system and conventional system is different enough that Reberg-Horton, Robarge and their colleague Shuijin Hu are wondering if one system is better than the other. “We don’t have enough info yet to determine that,” Reberg-Horton said.
In conventional farming, N2O emissions are driven by big preplanting fertilizer bursts. At NCSU’s experiment station in Goldsboro, researchers apply 75 pounds of nitrogen before planting corn, and Reberg-Horton said that’s the application that’s most vulnerable to losing nitrous oxide. They also apply 75 pounds of nitrogen as side dressing, which tends to stay put. “We think it has to do with the water,” said Reberg-Horton. “When we put it out pre-plant, you’ve got this tiny little plant, and it’s also the cooler part of the spring, so the soil can stay wet and that’s when we see nitrous oxide emissions. Later in the season, when we side dress, the plants are already big and it’s hard to keep the ground saturated. The big plants suck up the water so fast that we just don’t see as much N2O coming out of it.”
The results are most dramatic in crops that require more nitrogen fertilizer, specifically corn. Organics look a little different, and Reberg-Horton said this is because organic farming systems don’t allow large applications of nitrogen at any one time.
“We have a hypothesis, but we don’t have the data to support it yet,” said Reberg-Horton. “We’re still working on the data. In conventional systems we have so much excess nitrogen available that in the Southeast what regulates it is how much carbon is in the soil. The limiting factor is how much carbon is in the soil. In organic systems, the limiting factor is the amount of free nitrogen. The way each system fundamentally regulates how much N2O is emitted is different. It still leads to this question, which is frustrating: Which system emits more and less as you go all year-round? Or better yet, which system emits more when you go all the way through the [three-year] rotations of corn, soy and wheat?
Getting to theory
For Reberg-Horton, Robarge and their colleagues to develop their hypothesis, it required a mindset shift. First, they realized they had to adjust how nitrogen emissions are typically measured. In prior methods of sampling nitrogen emissions from soil, one uses hand syringes to pull samples from chambers laid on top of the soil. This method typically occurs weekly or during or soon after a rainstorm. “The when-to-sample issue was done much more cavalierly,” said Reberg-Horton.
The new method is more precise. “What we have is a trailer set up in the field plugged into a power pole that we installed out there. We’ve got vacuum pumps connected to these long hoses that go out into different parts of the field. Those vacuum hoses are pulling air from different chambers scattered throughout the field. The most essential part is the soil needs to be in the open – rained on and exposed to sun and everything else like normal soil, but then we need to periodically take gas sampling off that.”
To do that, the NCSU researchers use several robotic arms, each with a lid. The researchers drive something into the soil that each lid can sit on. Robarge developed a technique (and the machinery required) to connect this continuous sampling method with the old-fashioned chamber sampling method. ” As a result, Robarge, Reberg-Horton and their team now know not only exactly when to sample, but also how to plug the data into the larger set of information. “If there’s an event – like a two-day event that happens – if we go in and do just a couple of hand samples, we know where those samples fit in the entire pulse of events, so we can interpret the data differently.”
As yet, researchers have not figured out how to apply the data, but Reberg-Horton said it may lead to an alteration in fertilizer formulations. “We haven’t studied that yet, but anything that reduces the amount of free N being released all at once, in theory, will help,” he said. “The ideal would be that we could somehow alter our system management to help prevent [nitrous oxide emissions], and we don’t have good advice on that yet. There’s some very early evidence that there are other chemical compounds that are not used right now in the fertilizer industry that might inhibit N2O emissions. That would be promising.”