Study Points to Soil Fertilizer as the Likely Culprit of Central Valley Smog

SACRAMENTO, Calif. (CN) – California’s fertile Central Valley farm soil – and not the tractors and trucks working it – may hold the key ingredient of the state’s notoriously smoggy air, according to a University of California, Davis report.

The study published Wednesday in the journal Science Advances is the first large-scale report of California soil emissions and suggests that 25 to 41 percent of the state’s nitrogen oxide emissions are tied to fertilized agricultural soils.

Scientists say regulators have likely underestimated soil emissions’ impact on air quality and that the state and farmers should pursue ways to reduce the use of nitrogen fertilizers in California’s $45 billion agricultural industry.

“The effect of large soil nitrogen oxide emissions on air quality and human health remain unclear, but the magnitude of the flux alone raises concern about its potential impact, particularly in rural California,” the UC Davis-led report states.

Nitrogen oxide emissions, or NOx, are a major component of air pollution and are typically tied to the burning of fossil fuels. Poor air quality is a global problem, with the World Health Organization estimating that 1 in 8 premature deaths are related to smog.

While California has implemented myriad air quality and clean energy requirements over the last several decades, air quality in the rural Central Valley is routinely among the worst in the nation.

Despite sunny skies and 70-degree temperatures forecast through the weekend, on Wednesday the Central Valley Air Quality Coalition warned people between Fresno and Bakersfield not to “exercise or play outside today.”

The report notes that while the California Air Resources Board has conducted similar crop soil studies, it focused on farms near Sacramento and not the Central Valley, the most heavily fertilized and productive agricultural region in the state. The board attributes less than 4 percent of the state’s NOx emissions to cropland soils, with cars and airplanes being major contributors, the report highlights.

The air resources board did not immediately respond to requests for comment on the UC Davis study.

Benjamin Houlton, UC Davis professor and director of the John Muir Institute of the Environment, says the farming industry needs better ways to filter nitrogen from soils.

“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,” Houlton said in a statement.

The study, titled “Agriculture is a Major Source of NOx Pollution in California,” used airborne measurements from Fresno, Tulare and Kings counties along with statewide soil samples and computer model estimates. It recommends use of slow-release fertilizers to minimize the amount of nitrogen applied to crops, and other forms of fertilizer management.

According to the report, costs tied to nitrogen fertilizer waste total $210 billion in health and environmental damage annually.

“Reducing NOx emissions therefore offers a win-win situation for farmers, environmental health and the economy,” the report states.

Jim Houston, manager of governmental affairs for the California Farm Bureau Federation, said in a statement that California farmers are already taking action.

“Farmers have a long history of adjusting their practices in response to emerging science, and we will watch to see if further studies verify the results reported here. It’s important to note that most of the steps the study suggests are already underway. Farmers want to use the appropriate amounts of fertilizer and have long relied on expertise from the University of California in making those applications,” Houston said.

Research was funded in part by the National Science Foundation and the U.S. Environmental Protection Agency and was led by Houlton and professors from Northeast Normal University in China.

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