In the upper reaches of a Minnesota watershed, hydrologist Zhongjie Yu and his team observed something unusual: streams filled with water so saturated with nitrous oxide that it resembled a soda can just before popping its top. Nitrous oxide, a potent greenhouse gas, is typically found in much lower concentrations in natural waters, but in certain agricultural areas, the levels can be tens of thousands of times higher than would be expected in equilibrium with the atmosphere. This discovery prompted Yu, an assistant professor at the University of Illinois Urbana-Champaign, to probe deeper into the source of this excess nitrous oxide and its implications for greenhouse gas emissions.
Yu’s work, published in two recent studies in Environmental Science & Technology and Geophysical Research Letters, sheds new light on the complex dynamics of nitrous oxide emissions in agricultural regions. While it is widely acknowledged that agriculture, particularly nitrogen-based fertilizers, is a significant contributor to global nitrous oxide emissions, Yu’s research suggests that traditional methods of measuring emissions from soil alone may be overlooking a critical factor: the emissions from downstream ecosystems, including streams and rivers. These emissions, the study found, could account for up to one-third of the total nitrous oxide emissions in the Corn Belt region of the United States—an area known for its intensive agricultural practices.
Nitrous oxide, which traps heat nearly 300 times more effectively than carbon dioxide, is a long-lived greenhouse gas in the atmosphere. Its concentration is not only influenced by direct emissions from agricultural practices but also by indirect emissions that occur when nitrogen compounds in the soil are transported by runoff into streams. In these waterways, the nitrogen undergoes microbial processes such as nitrification and denitrification, releasing nitrous oxide into the environment. Understanding the role of streams in the nitrogen cycle is essential for refining existing models of regional greenhouse gas budgets and for designing more effective mitigation strategies.
The process of measuring nitrous oxide emissions has typically focused on direct measurements from agricultural soils, often via chambers placed on the surface to capture emissions. However, Yu’s research indicates that this method is insufficient for capturing the full scope of emissions, particularly those occurring downstream. These “indirect” emissions from rivers and streams, which carry nitrogen from agricultural soils, have been largely ignored in previous models. By tracing the sources of these emissions, Yu and his collaborators were able to show that they could make up a significant portion of the overall emissions in certain regions.
Yu explains that while it is relatively straightforward to measure the total nitrous oxide emissions in a given year—since the gas is persistent in the atmosphere—identifying where the emissions originate from is more complex. It is well-established that agriculture, through the use of nitrogen fertilizers, contributes to nitrous oxide emissions. When farmers apply fertilizer, some of it is absorbed by crops, some is lost to nearby waterways, and some undergoes microbial transformation in the soil, producing nitrous oxide. What has been less well understood is how much of the gas is transported via water runoff to streams and rivers, where it is subsequently emitted into the atmosphere.
The new findings suggest that a more accurate assessment of regional nitrous oxide emissions requires a more holistic approach that incorporates both soil and water systems. By understanding how nitrogen from agriculture interacts with waterways, scientists can refine emission estimates and provide more effective guidance for reducing greenhouse gas emissions. Yu argues that stream emissions are particularly significant during specific “hot moments” and “hot spots,” such as after the application of ammonia-based fertilizers followed by intense rainfall events. During these times, runoff can carry high concentrations of nitrous oxide into rivers, where it is released into the air.
In their study, the researchers used sophisticated isotopic analysis to track the origins of the nitrous oxide in streams. The analysis revealed that up to half of the nitrous oxide in the streams they studied came from nitrification in agricultural soils. The team also found that stream emissions were highest in areas with strong connections between streams and surrounding soils, particularly during wetter periods when large storm events or snowmelt occur. These findings suggest that certain geographic areas and times of year contribute disproportionately to nitrous oxide emissions, highlighting where targeted mitigation efforts could be most effective.
Yu’s research also examined how these emissions correlate with atmospheric measurements taken from a tower 328 feet above the ground. Using isotopic signatures from the air, the team estimated that at least 35% of the nitrous oxide in the region originated from streams. While this finding is based on data from only one tower in Minnesota, it suggests that stream emissions could be a much larger factor in nitrous oxide budgets than previously thought. Yu and his collaborators plan to expand their study, using a network of seven towers across a broader area, to further investigate the significance of stream emissions.
This new understanding of the role of streams in the nitrogen cycle has important implications for agricultural management. Practices that reduce nitrogen leaching or promote efficient water recycling not only improve water quality but also have the potential to reduce greenhouse gas emissions. For example, incorporating winter cover crops or adopting controlled irrigation techniques can help minimize runoff and improve nitrogen retention in the soil, reducing the amount of nitrogen that reaches streams. Conversely, certain soil management practices that enhance water infiltration—while beneficial for preventing waterlogging—could inadvertently increase nitrous oxide emissions from downstream waterways.
The research underscores the importance of considering both nitrogen and water cycles together when developing agricultural management strategies. Traditional approaches that focus solely on improving nitrogen use efficiency may overlook the significant role that water plays in transporting nitrogen into streams, where it can be converted into nitrous oxide. As such, Yu advocates for a more holistic approach to managing agricultural systems that considers the interactions between soil, water, and nitrogen to mitigate greenhouse gas emissions more effectively.