Researchers have discovered that the emissions from streams are primarily generated by nitrification processes occurring in agricultural soils. They also found that these stream emissions contribute a far larger share to the annual nitrous oxide emissions than was previously understood.
In the upper areas of a Minnesota watershed, the water contains such high levels of dissolved nitrous oxide that University of Illinois Urbana-Champaign hydrologist Zhongjie Yu compares it to a can of soda.
“If you collect water from local streams and analyze the nitrous oxide levels, the concentration is tens of thousands of times higher than what would be expected in equilibrium with the atmosphere. Essentially, it’s extremely saturated with this powerful greenhouse gas. Naturally, one wonders about its source,” explained Yu, an assistant professor in the Department of Natural Resources and Environmental Sciences at Illinois.
In two recent studies, Yu and his team discovered that the nitrous oxide emissions from streams like those sampled in Minnesota mainly result from nitrification in agricultural soils. Additionally, they determined that these stream emissions account for a significantly larger portion of the yearly nitrous oxide budget than was previously acknowledged.
“Typically, the method for estimating nitrous oxide emissions involves using chambers placed on the soil surface, but focusing solely on soil doesn’t reveal emissions from downstream or downwind ecosystems that absorb excess nitrogen escaping from agricultural practices,” Yu noted. “When we traced those downstream emissions, we found that they could represent up to one-third of the total nitrous oxide emissions in the Corn Belt region.”
It’s relatively easy to quantify total nitrous oxide emissions for a year. This greenhouse gas, which is nearly 300 times more effective at trapping heat than carbon dioxide, persists in the atmosphere for a long time. Thus, if scientists measure it at one point in time, they can readily calculate the amount that has accumulated over subsequent seasons or years. However, pinpointing its sources is more complicated.
It is widely acknowledged that agricultural activities are a significant contributor to atmospheric nitrous oxide. When farmers apply nitrogen-based fertilizers, some is absorbed by crops, some washes into nearby streams, and some is converted by soil microbes into nitrous oxide. This gas can escape into the atmosphere quickly, or it can dissolve in soil moisture and eventually reach groundwater or streams during rainfall or snowmelt.
Yu emphasizes that the nitrous oxide stored in soil, transported via runoff, and released from receiving streams and rivers has been often neglected in discussions about agriculture’s contribution to greenhouse gas emissions.
“By gaining a better understanding of these indirect stream emissions, we can also improve the accuracy of direct soil emission estimates. Our findings suggest that the substantial contribution from stream emissions indicates that soil emissions might have been overestimated in existing estimates of regional nitrous oxide emissions,” Yu explained. “Creating a solid regional inventory of nitrous oxide emissions is a crucial first step in developing effective mitigation strategies and assessing their outcomes across larger areas.”
During the microbial processes of nitrification and denitrification, nitrogen and oxygen—the components making up nitrous oxide—experience slight isotopic changes that Yu’s instruments can detect, similar to a unique fingerprint. When his team analyzed stream water, they found that nearly half of the nitrous oxide originated from nitrification happening in agricultural soils. Their analysis also identified what Yu describes as “hot spots” and “hot moments” of nitrous oxide production and subsequent discharge into waterways, particularly after ammonia-based fertilizers are applied, followed by heavy rain.
“Our findings indicate that stream emissions peak in regions with strong connections between streams and surrounding soils, especially during wet conditions. Major storm events, melting snow, and the installation of drainage tiles, which improve connections between soil and streams, greatly enhance nitrous oxide emissions through waterways,” he stated. “These locations and occurrences should be prioritized for focused mitigation efforts.”
When the research team sampled air from a tall tower standing 328 feet above the ground, isotopic signatures showed that at least 35% of the region’s nitrous oxide emissions originated from streams. However, Yu is careful not to overemphasize this estimate, as it was based on observations from a single tower in Minnesota. He and his colleagues plan to expand their sampling across a broader area using a network of seven towers soon. Nevertheless, the finding suggests that scientists and land managers ought to pay closer attention to streams linked to agricultural activities.
“In discussions regarding nitrous oxide emissions from agriculture, the focus often rests on nitrogen fertilizer inputs or poor nitrogen use efficiency within farming systems. However, the findings from our research broaden our understanding of the indirect emission pathways through streams and rivers,” Yu said. “This indicates that management practices aimed at reducing nitrogen leaching or promoting efficient water recycling are not only beneficial for improving water quality but could also help decrease greenhouse gas emissions from heavily farmed regions. For instance, employing winter cover crops in rainfed fields or controlled irrigation practices can be effective strategies.
“Conversely, practices that enhance soil water infiltration—generally seen as beneficial for preventing soil saturation—may unintentionally lead to increased nitrous oxide emissions downstream,” he added. “This underscores the necessity for comprehensive management strategies that consider both nitrogen and water cycles simultaneously.”
