Farms in the U.S. Contain Chemical-Filled Soil Responsible for a Shocking Amount of Air Pollution

A grove of young pistachio trees near Porterville, California, August 24, 2016. Fertilized soils are releasing nitrogen oxide, a dangerous pollutant that is believed to contribute to smog formation. Robyn Beck/AFP/Getty Images

Air pollution was responsible for one in eight deaths across the world in 2012. But most likely associate this smoggy mess with smokestacks, cars spitting out black smoke, or massive cities. There is, however, an unlikely source of pollution coming from right beneath your feet. A new study in California revealed that more air pollution than we previously thought is emanating from the soil.

The study, published Wednesday in Science Advances, found that between 25 and 41 percent of nitrogen oxide is coming from agricultural lands in California. "We were sort of blown away by the number at first because California has always thought of emissions from soils as being very low," Maya Almaraz, postdoctoral fellow at University of California, Davis, told Newsweek.

Smog-filled skies in California aren't only within the haze that sits over Los Angeles on a hot summer day—agricultural lands in the Central Valley are also emitting air pollution due to a combination of excess fertilizer, hot and dry climate conditions and the geographical conditions. Fertilizers are food for crops, but only half of the nitrogen in fertilizers is eaten up by the plants. The remainder sinks into the soil, where microbes digest nitrogen and spark the release of a waste product in the form of nitrogen oxide, or NOx.

Maya Almaraz, a National Science Foundation postdoctoral fellow at University of California Davis, samples soils for nitrogen oxide emissions in Palm Springs, California in January, 2018. Almaraz is the lead author of a new study that found nitrogen oxide emissions from agricultural lands are higher than previously understood. Maya Almaraz/UC Davis

Nitrogen oxide on its own is only dangerous at high concentrations. The reason it becomes deadly for humans is the way it contributes to smog, or a form of pollution called ground level ozone. Ground level ozone is associated with heart disease, asthma and other respiratory illnesses.

"We don't know in our study how much of this NOx is converted into smog, and we're working on that question right now," Ben Houlton, senior author and director of UC Davis's John Muir Institute of the Environment, told Newsweek. While scientists understand the deadly impacts of air pollution from smog, ozone and other pollutants, this odd source is more difficult to grasp.

"Since you can't see it coming from the soils, it's very easy to miss," Houlton said. "It's very easy to say, 'Well, I don't imagine these soils and systems are producing pollution because I always think that pollution comes from vehicles or fossil fuel combustion or smokestacks.'"

Despite the seemingly strange source, fertilized soils create the "perfect storm of conditions" to produce the nitrogen oxide. In valleys where many California crops are grown, air gets stuck in what Houlton described as a "bathtub," after sweeping over the mountain ranges from the West. The air sits in the valley low to the ground, where hot and dry conditions mixed with increased amounts of pollution from fertilized soils can result in smog formation.

Reducing this previously underestimated source of air pollution is a challenge, as we need fertilizer to grow food. "We absolutely need to grow food, and California does a great job of doing that," Almaraz said. Half of fruits and nuts produced in the U.S. are grown in the Central valley, according to researchers, and that includes much of the nation's almonds, walnuts, raisins, avocados and tomatoes. The fertilizer used for that food, however, is largely wasted. Only half of the fertilizer is processed by plants—the rest sinks into soils and water sources.

"Improving that number is really important, and that can really help farmers because it saves them money," Almaraz said. Technologies to improve fertilizer management, such as slow-release fertilizers or agricultural practices to improve water and fertilizer efficiencies could help, according to researchers.

"Smog from soil might be bad news to people," Almaraz said. "It's also an opportunity to deal with some of these air quality issues that [exist] in California."