Crops May Contribute More to California’s Smog Than Previously Thought

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Lead author Maya Almaraz samples soils for NOx emissions in Palm Springs in January 2018. (Courtesy UC Davis/Maya Almaraz)

New research from UC Davis finds a sneaky contributor to the state’s smog problem rising from the floor of the Central Valley: California’s crops are emitting polluting nitrogen oxides, up to 10 times as much of those gases as previously thought.

State regulators blame nitrogen oxides, also commonly known as NOx, for health problems and even deaths. Over decades, the California Air Resources Board and scientists have worked to inventory NOx gases. Most often, those pollutants are connected to burning fossil fuels, tailpipes and smokestacks.

Agriculture is another source of nitrogen oxides. Fertilizers add nitrogen to soil; what plants don't use gets digested by microbes, and soil and air conditions can intensify that process. A study published in "Science Advances" finds that those emissions may make up 25 to 41 percent of the state’s total.

“We have been able to make big strides in improving air quality in cities,” says National Science Foundation postdoctoral fellow Maya Almaraz, the study’s lead author. “But we’re just not seeing those changes happen as quickly in rural areas, which we think might be because we have this sort of undetected source in those areas."

In the past, the state Air Resources Board has used satellites and local field studies to characterize the pollutant. Almaraz and her team relied on new and different methods, including soil calculations and data collected by low-flying planes to create their calculations.


The new results differ sharply from the current state air pollution inventory, last updated in 2013, which has put farm-based NOx pollution at 3.8 percent.

Man on a tractor.
A farmer plows a field on Aug. 11, 2004, near the town of Arvin, southeast of Bakersfield, California. (David McNew/Getty Images)

“These numbers seem to be quite a bit different from our prior understanding,” says Bart Croes, head of research for the California Air Resources Board. “So we’re going to take a close look at their methods and see if we need to adjust our understanding.”

But Croes says he has questions about the new study’s methodology. “When we look at satellite data, it shows that the remaining hot spots of pollution in California are, as you’d expect, our major cities,” he says. “So that seems to align with the prior understanding that we have, that cars and trucks and other urban sources are responsible for the pollution we need to control.”

Almaraz and Croes both acknowledge uncertainties in measuring the pollutant in farm fields. The microbial process that produces nitrogen oxides is a pulsing, complex one, dependent on multiple factors including fertilizer application, temperature, air and soil moisture.

Almaraz and her co-authors point out that their results are on par with agricultural emissions described in other studies for Europe and the Midwest.

“Looking back we realize, oh, of course, perhaps it isn’t as surprising as we first thought that we’re getting these large soil emissions of NOx,” Almaraz says. “We’re seeing this elsewhere in the world, and it was just a matter of time before we looked at it in California.”

The study’s results may have longer-term implications for California farmers. Regulations aimed at cutting smog have for decades been largely focused on mobile sources, like cars and trucks, and major stationary sources, like refineries. California already incentivizes less-polluting farming practices and promotes strategies to cut emissions from crops, such as through slow-release, efficient fertilizing practices.

Those strategies may not be enough going forward. Almaraz points out that a growing population is likely to demand more produce -- which means farm-related emissions may grow in significance.

“In California we love and respect our agricultural sector,” she says. “We hope we can help find solutions for environmental health and agricultural production, too.”