It's long been understood that the valley portion of Kern County suffers from some of the worst air in the nation.
It also grows the most valuable agricultural products in the country.
This week those two facts converged when a study led by researchers from UC Davis found that farm fields up and down the San Joaquin Valley are emitting huge amounts of a gas that is a major contributor to the valley's smog problem.
The study, published Wednesday in the research journal Science Advances, concludes that unused fertilizer in agricultural soils contributes 25 to 41 percent of nitrogen oxide, or NOx, emissions in California.
In the study, the authors compared computer models with estimates collected from scientific flights over the San Joaquin Valley. Both the model and flight data suggested that soils with heavy nitrogen fertilizer applications are the source of NOx emissions, a precursor to ozone smog.
But when nitrogen-based fertilizers are used efficiently — when crops absorb the nitrogen rather than leaving significant amounts in the soil — that soil does not become a source of NOx in the air.
"We need to increase the food we’re making," said the study's lead author Maya Almaraz, a National Science Foundation postdoctoral fellow in UC Davis Professor Ben Houlton’s lab. "We need to do it on the land we have. But we need to do it using improved techniques."
The Central Valley is one of the world’s most highly productive agricultural areas. Roughly half of the fruits and nuts produced in the United States are grown here, including nearly all the nation’s almonds, walnuts, raisins, avocados and tomatoes.
Beatris Espericueta Sanders, executive director of the Kern County Farm Bureau, said farmers are always working to identify best practices to grow food for the world and improve air quality.
"We often consider UC Davis studies to adjust our practices," Sanders said in an email. "This study raises some initial questions that I believe our industry would be interested in reviewing in more detail to assess its impact to growers in Kern County. It's important to note that most of the steps the study suggests are already underway and we are always open to working with partners to make our industry more efficient."
Tom Frantz, a Shafter almond grower who works as a clean air advocate said NOx is also a precursor to ammonium nitrate which represents a significant portion of the region's PM 2.5 pollution.
"The imported, inorganic nitrogen fertilizers are the problem," he said. "We have to recycle and compost more. Biomass, such as wood waste needs to be returned to the soil."
UC Davis' Houlton, who described himself as "100 percent pro-farming" and whose dad was a farmer in the Midwest, told The Californian he thinks many growers are already working to reduce residual nitrogen in the soil.
Excess nitrogen in the soil has long been recognized as a water problem. It wasn't really a big stretch to recognize that it is getting into the air as well.
"This is not to blame farmers," Houlton said. "We know we need to be more efficient in fertilizer use."
Sometimes it's easier to simply apply fertilizer, rather than measuring the nitrogen already in the soil.
"Some farmers are already forming coalitions to know how much fertilizer to use," he said.
Smog-forming nitrogen oxides, or NOx, are a family of air-polluting chemical compounds. They are central to the formation of ground-level ozone and contribute to adverse health effects, such as heart disease, asthma and other respiratory issues.
Fossil fuels have long been recognized as a major contributor to NOx pollution, researchers said in the study. Technologies such as the catalytic converter have helped greatly reduce NOx emitted from vehicles in urban areas. But some of the state’s worst air quality problems are now in rural areas, particularly the San Joaquin Valley.
The study suggests potential solutions for reducing NOx soil emissions, primarily through different forms of fertilizer management.
Only about half of the nitrogen fertilizer applied to crops is used by the plant, the study said. But slow-release fertilizers that deliver nutrients in a way that more closely mimics nature have been shown to greatly improve nitrogen use efficiency of crops, reducing emissions of nitrogen in the environment.
“Since this source of NOx can remain local, largely in rural farming communities, we need to develop a kind of catalytic converter for soils and farms," Houlton said. "It’s critical that new policies focus on incentives to bring the latest nutrient management technologies to farms so that growers can produce food more efficiently, increasing their bottom line and improving rural health."