Streams near farms are 'fizzy' with nitrous oxide
In the serene, upper reaches of a Minnesota watershed, the water’s chemical makeup holds a startling secret. The streams are so saturated with dissolved nitrous oxide that hydrologist Zhongjie Yu of the University of Illinois Urbana-Champaign compares the water to fizzy soda from a can.
“If you grab water from the local streams and measure nitrous oxide, the saturation is tens of thousands of times higher than it would be at equilibrium with the atmosphere. In other words, it’s super-saturated with this potent greenhouse gas. Naturally, you wonder where it’s coming from,” explained Yu.
Annual nitrous oxide in streams
Yu and his collaborators explored this phenomenon in two recent studies, and revealed that emissions from such streams stem primarily from nitrification in agricultural soils.
Their findings suggest that these stream emissions contribute significantly more to the annual nitrous oxide budget than previously estimated.
“The conventional method to estimate nitrous oxide emissions is to measure from a chamber placed on the soil surface, but focusing entirely on soil doesn’t give you any idea of nitrous oxide emissions from downwind or downstream ecosystems that receive excess nitrogen lost from agricultural systems,” explained Yu.
“When we traced those downstream emissions, we found they could potentially account for one-third of the total nitrous oxide emissions within the Corn Belt region.”
Greenhouse gas with hidden pathways
Nitrous oxide is a powerful greenhouse gas that traps heat almost 300 times more effectively than carbon dioxide. It’s closely linked to agriculture, where nitrogen-based fertilizers are used to help crops grow.
The problem is that these fertilizers don’t just stay in the soil – they can release nitrous oxide into the air or leach into nearby streams.
Soil microbes play a role too, turning excess nitrogen into nitrous oxide. Sometimes the gas escapes right away, but at other times it gets trapped in the soil or dissolves in water, only to be released later during rain or snowmelt. It’s a hidden cycle that impacts both the atmosphere and waterways.
Despite the acknowledged role of agriculture, researchers have historically overlooked nitrous oxide emissions from streams and rivers connected to these systems.
“By better understanding these indirect stream emissions, we can refine estimates of direct soil emissions,” said Yu. “In our case, the high contribution of stream emissions suggests that soil emissions may have been overestimated in current regional nitrous oxide budgets.”
Tracing the sources of nitrous oxide
Yu’s team studied nitrogen and oxygen isotopes in nitrous oxide to find its source. They found that up to half of the nitrous oxide in the streams came from agricultural soil nitrification.
The researchers also identified “hot spots” and “hot moments” of emissions, particularly when the application of ammonia-based fertilizers is followed by heavy rainfall.
“Our results suggest that stream emissions are highest in areas with strong connections between streams and surrounding soils, particularly during wetter periods,” Yu noted.
“Large storm events, snowmelt, and installations of tile drains, which enhance soil-stream connections, contribute disproportionately to high nitrous oxide emissions via streams. These areas and events should be prioritized for targeted mitigation efforts,” expanded Yu.
Broader research is needed
Using air samples collected from a 328-foot-tall tower in Minnesota, Yu’s team found that at least 35% of regional nitrous oxide emissions originated from streams.
Although this estimate came from a single tower, the findings highlight the need for broader research across a network of sites. Yu and his collaborators plan to expand their studies to a seven-tower network, aiming to capture a more comprehensive picture.
“When discussing agricultural nitrous oxide emissions, the focus is often on fertilizer nitrogen inputs or low nitrogen use efficiency in agricultural systems,” Yu explained.
“The results from our studies broaden this understanding by revealing the potential importance of indirect emission pathways via streams and rivers.”
Mitigation strategies for a complex problem
Addressing this issue requires integrated approaches that consider both nitrogen and water cycles.
Practices such as planting winter cover crops in rainfed fields or adopting controlled irrigation could reduce nitrogen leaching and emissions. However, not all strategies are without risk.
“On the other hand, practices that promote enhanced soil water infiltration – generally regarded as beneficial for preventing water-logged soil conditions – could inadvertently increase downstream nitrous oxide emissions,” Yu cautioned. “This highlights the need for holistic management approaches.”
Future research on nitrous oxide in streams
The two studies, “Hydrologic connectivity regulates riverine N2O sources and dynamics,” and “Isotopic constraints on nitrous oxide emissions from the U.S. Corn Belt,” highlight the importance of revisiting nitrous oxide budgets.
By including overlooked emission pathways from streams, policymakers and land managers can craft more effective mitigation strategies.
Yu’s findings not only challenge existing models but also emphasize the critical role of interdisciplinary research in tackling greenhouse gas emissions.
As scientists continue to uncover the hidden pathways of nitrous oxide, a more comprehensive understanding of agricultural emissions could pave the way for innovative solutions. These efforts promise not only to improve water quality but also to combat the escalating challenge of climate change.
The studies are published in the journals ACS Publications and Geophysical Research Letters.
—–
Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.
Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.
—–
