image: In two recent papers led by the University of Illinois Urbana-Champaign, researchers found that emissions from streams are largely derived from nitrification processes in agricultural soils. Further, they found that stream emissions make up a much greater portion of the annual nitrous oxide budget than previously known.
Credit: Zhongjie Yu, University of Illinois Urbana-Champaign
URBANA, Ill. -- In the upper reaches of a Minnesota watershed, the water is so full of dissolved nitrous oxide that University of Illinois Urbana-Champaign hydrologist Zhongjie Yu likens it to a soda 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,” said Yu, an assistant professor in the Department of Natural Resources and Environmental Sciences, part of the College of Agricultural, Consumer and Environmental Sciences at Illinois.
In two recent papers, Yu and his collaborators found that emissions from streams like the one they sampled in Minnesota are largely derived from nitrification processes in agricultural soils. Further, they found that stream emissions make up a much greater portion of the annual nitrous oxide budget than previously known.
“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,” Yu said. “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.”
Backing up, it’s relatively straightforward to measure the total nitrous oxide emissions in a given year. The greenhouse gas, which traps heat nearly 300 times more efficiently than carbon dioxide, is long-lived in the atmosphere. So if scientists measure it at one time point, it’s easy to calculate how much more has accumulated in the following season or year. What’s more challenging is determining where it’s coming from.
It is generally accepted that agriculture is a major source of nitrous oxide in the atmosphere. When farmers apply nitrogen-based fertilizer, some is taken up by crops, some is lost to nearby streams, and some is transformed by soil microbes into nitrous oxide. The gas can escape into the air immediately or, if it is trapped belowground in soil pores, it can dissolve into soil water and eventually be carried to groundwater and/or streams during rain or snowmelt events.
Yu says the portion of nitrous oxide that is stored in soil, transported with runoff, and emitted from receiving streams and rivers has been overlooked as part of agriculture’s contribution to greenhouse gas emissions.
“By better understanding these indirect stream emissions, we can refine estimates of direct soil emissions. In our case, the high contribution of stream emissions suggests that soil emissions may have been overestimated in current regional nitrous oxide budgets,” Yu said. “Establishing a robust regional nitrous oxide emission inventory is an important first step in designing effective mitigation strategies and verifying their outcomes at large spatial scales.”
During the microbial processes of nitrification and denitrification, nitrogen and oxygen — the components of nitrous oxide — take on subtle isotopic changes that Yu’s instruments can read like a fingerprint. When his team sampled stream water, they could tell that up to half of the nitrous oxide came from the process of nitrification in agricultural soils. The analysis also revealed what Yu calls “hot spots” and “hot moments” of nitrous oxide production and subsequent loss to waterways, such as after the application of ammonia-based fertilizers, followed by intensive rainfall events.
“Our results suggest that stream emissions are highest in areas with strong connections between streams and surrounding soils, particularly during wetter periods. Large storm events, snowmelt, and installations of tile drains, which enhance soil-stream connections, contribute disproportionately to high nitrous oxide emissions via streams,” he said. “These areas and events should be prioritized for targeted mitigation efforts.”
When the team sampled air from a tall tower 328 feet above ground, isotopic signatures revealed that at least 35% of nitrous oxide in the region came from streams. Yu is cautious not to overstate that estimate, however, given that it came from only one tower in Minnesota. He and his collaborators plan to sample a greater area across a seven-tower network in the near future. Still, the finding suggests scientists and land managers should pay more attention to ag-connected streams.
“When discussing agricultural nitrous oxide emissions, the focus is often on fertilizer nitrogen inputs or low nitrogen use efficiency in agricultural systems. The results from our studies broaden this understanding by revealing the potential importance of indirect emission pathways via streams and rivers,” Yu said. “This means that management practices that reduce leaching or promote efficient water recycling are not only best management practices for improving water quality, but also have the potential to lower greenhouse gas emissions from intensively agricultural regions. For example, incorporating winter cover crops in rainfed fields or controlled irrigation can be effective strategies.
“On the other hand, practices that promote enhanced soil water infiltration (that is, water movement via soil profiles), which is generally regarded as beneficial for preventing water-logged soil conditions, could inadvertently increase downstream nitrous oxide emissions,” he added. “This highlights the need for holistic management approaches that consider both nitrogen and water cycles in tandem.”
The first study, “Hydrologic connectivity regulates riverine N2O sources and dynamics,” is published in Environmental Science & Technology [DOI: 10.1021/acs.est.4c01285]. The second study, “Isotopic constraints on nitrous oxide emissions from the US Corn Belt,” is published in Geophysical Research Letters [DOI: 10.1029/2024GL109623].
Both studies were supported by the National Science Foundation [projects 2110430 and 2110241]. Additional sources of funding included the USDA-NIFA (projects ILLU-875-983 and 2018-67019-27808), the National Natural Science Foundation of China (project 42107393), and the European Commission under the Horizon 2020 Research and Innovation Framework Programme (ATMO-ACCESS 101008004).
Journal
Geophysical Research Letters