Nitrous Oxide, a Potent Greenhouse Gas, Is Spreading From Farms

Nitrous oxide (N₂O) emissions from agriculture are not only coming from soil. They’re also flowing through the streams that run alongside farms, a recent study finds. Researchers at the University of Minnesota and University of Illinois discovered more than one-third of regional nitrous oxide emissions were measured from streams.

While often overshadowed by the more widely known greenhouse gases like carbon dioxide and methane, nitrous oxide is the third-largest contributor to greenhouse gas emissions. Nitrous oxide also has an EPA-rated “Global Warming Potential” 273 times that of carbon dioxide (over a 100-year timescale), and is a significant contributor to ozone depletion. This means that even small amounts of nitrous oxide emissions can have a disproportionately large impact on global warming. And the flip side is also true: taking some action, including better manure management and eating less meat, can have a big climate impact for the better.

The discovery highlights a previously overlooked pathway for nitrous oxide to enter the atmosphere: as agricultural soils undergo the process of nitrification and produce nitrous oxide, the gas is transported via runoff into nearby streams. During the nitrification process, bacteria in soils break down ammonia — in this case from fertilizers — into nitrates that can produce nitrous oxide gas. While this gas may be released immediately from the soil, it can also become trapped in the soil or dissolve in water, only to be carried into streams during rainfall or snowmelt.

 “What we basically found is there [are] other pathways, and this stream emission could be important,” Zhongjie Yu, hydrologist and researcher in the study, tells Sentient.

Traditionally, when researchers consider nitrous oxide emissions from agriculture, they focus on soil. Excess nitrogen from fertilizers accumulates in agricultural soil, where it can be transformed into nitrous oxide,  and released directly into the atmosphere. However, this new study suggests that emissions from streams — carrying runoff from agricultural fields — can be just as important, if not more so, in certain regions. Factory farms can be a major driver of this issue, as these operations produce large amounts of waste, composed partly of nitrogen.

When researchers consider initial nitrous oxide emissions, it’s often in the context of fertilizer input to soil. This excess nitrogen accumulates in agricultural soil, and can then lead to the direct release of nitrous oxide into the atmosphere.

Though the study used only the University of Minnesota’s Tall Tower Trace Gas Observator to sample air emissions, Yu and his research team are confident about the data.

“We found that N₂O is emitted from streams, but we believe it’s originally from fertilizer nitrogen. It is just carried by water, and gets emitted from those aquatic systems. It’s not saying it’s a natural source, but it’s still related to fertilizer nitrogen. But it just provides another pathway that N₂O gets lost from some land to the atmosphere.”

Yu also emphasized that agriculture is the dominant source of human-caused nitrous oxide emissions. This study not only challenges the conventional understanding of agricultural emissions, but also underscores a broader issue: agricultural runoff is not just a water quality problem — it is also a major contributor to climate change.

Nitrous Oxide: the Lesser Known Climate Threat

Human activities are significant contributors to nitrous oxide emissions. While other natural sources, such as the ocean, also release nitrous oxide, agricultural practices — particularly fertilizer application — are by far the largest contributor of anthropogenic emissions.

This gas is primarily produced when nitrogen-based fertilizers and manure are applied to agricultural soils. Microbes in the soil convert this nitrogen into nitrous oxide through processes such as nitrification and denitrification. The situation is made worse by the practices of industrial farming, which rely heavily on the use of synthetic fertilizers and animal waste.

Around 12 million tons of nitrogen are applied to crops each year in the United States. An EPA report found that agricultural soil management — like fertilizer use and other practices that boost nitrogen levels in the soil — accounts for nearly 75 percent of the United States’ nitrous oxide emissions. Within the agricultural sector itself, it makes up almost half of all emissions.

These soil management practices are primarily intended to optimize crop production, but much of the land is used for growing farm animal feed and biofuel crops. In 2020, only a small fraction of corn grown in the U.S. — less than two percent — was used for direct human food. Around  31 percent was dedicated to biofuels such as ethanol, while the majority, around 35 percent, went toward feeding livestock. With global meat consumption steadily rising, the demand for animal feed is expected to increase, further driving the need for feed crops like corn — along with higher fertilizer use and a corresponding rise in nitrous oxide emissions.

And now, the source of these emissions isn’t just the soil. Stream emissions can contribute to a much larger portion of annual nitrous oxide emissions than researchers previously thought.

“The high contribution of stream emissions suggests that soil emissions may have been overestimated in current regional nitrous oxide budgets,” Yu writes.

Yu points out that the link between agricultural runoff and stream emissions has often been overlooked in past research. Traditional methods use soil chambers to measure nitrous oxide emissions, a method that places a closed chamber over the soil, in order to capture any gases emitted from it. This approach primarily measures how much nitrous oxide is released directly from the soil, but misses the nitrogen that leaches out of the soil and flows into water systems, where it can then be converted into nitrous oxide and emitted to streams.

The Bottom Line

The source of nitrous oxide emissions from agriculture is more complex than scientists once thought. While traditional efforts to reduce nitrous oxide emissions have focused mainly on fertilizer management and improving soil efficiency, this new discovery emphasizes the importance of looking at the entire agricultural system, including runoff into streams. If the number of factory farms continue to rise, there is likely to be a rise in nitrous oxide pollution, particularly in streams, due to greater fertilizer use and runoff.

Yu mentions that improving water and nutrient management not only benefits water quality, but can also be a crucial tool in reducing greenhouse gas emissions. Fertilizer and manure runoff, especially from factory farms, can carry large quantities of nitrogen into waterways, which may then be converted into nitrous oxide — further complicating efforts to mitigate emissions from agriculture.

“We need to be a little bit careful here, because that water gets leached from surface soil and can end up in streams and rivers, carrying a lot of that greenhouse gas to those systems and having an impact there,” Yu says.