Where greenhouse gases come from
Think of California’s smog problem and you probably think of tailpipes and smokestacks.
A startling new study led by UC Davis, however, says the fertilizer in farm soils is a major contributor to smog in California.
In a study published Wednesday in the research journal Science Advances, a team led by UC Davis says soils, fertilized and natural, contribute 25 to 41 percent of nitrogen oxide emissions in California.
While natural soils emit nitrogen oxide, too, the concentration is far higher in fertilized cropland, the researchers wrote. Nitrogen oxide, or NOx, is a major factor in smog formation.
“Emissions of NOx from agricultural areas are much higher than we used to believe and could be a major source of atmospheric NOx statewide,” the researchers wrote in the article. The article, written with several researchers from China, is titled, “Agriculture is a major source of NOx pollution in California.”
The findings are especially provocative in light of UC Davis’ long-standing relationship with California agriculture. UC Davis researchers have developed world-renowned plant varieties and a multitude of new technologies for farmers. Among the most famous is the mechanical tomato harvester, invented at UC Davis in the 1950s, which revolutionized California’s tomato industry. The university’s sports teams are called the Aggies.
Cannon Michael, a prominent farmer from the Los Banos area, said that it didn’t take long for him to receive a copy of the report as it was passed around by frustrated members of the agricultural community. Michael said he fears the study will lead to more environmental regulations, which could make it harder for growers to compete with farmers in other, less-regulated states.
That the study came from UC Davis was especially frustrating to Michael.
“The university has made very few friends over the last few years in the agricultural community,” Michael said. “I get grief sometimes ... for attending UC Berkeley as my alma mater, but I would posit that Davis has occupied just as active an environmental role, or even a negative role, in terms of how agriculture has been portrayed through some of their reports and studies.” He said he has previously spoken to UC Davis officials about research that he says unfairly portrayed California’s $45 billion-a-year farm industry negatively.
The study’s co-author Benjamin Houlton, a professor at UC Davis’ department of land, air and water resources, said in an interview that the researchers are prepared for some blowback from farm groups. But he also thinks most farmers will accept the findings.
“It’s always something of a tightrope in these situations,” Houlton said. “But we feel very confident in our results.”
Houlton said the study was based on air-quality samples conducted in Fresno, Tulare and Kings counties, soil samples from throughout the state, and computer models. “We’re left with the logical conclusion that this is a big source (of NOx),” he said.
He added that the findings are comparable to earlier studies done in the Midwest, Europe and China. He said he believes farmers are prepared to find new growing methods that reduce reliance on nitrogen fertilizers.
“We need to increase the food we’re making. We need to do it on the land we have. But we need to do it using improved techniques,” said Maya Almaraz, the study’s lead author and a postdoctoral fellow at UC Davis, in a prepared statement.
Jim Houston, manager of governmental affairs for the California Farm Bureau Federation, said in a statement: “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.”
The report noted that previous studies by state officials showed that cropland has been an insignificant factor in smog formation. The California Air Resources Board has said 83 percent of NOx emissions come from motor vehicles, airplanes and other “mobile sources.”
The estimates showing negligible contributions from crop soil “are based on data limited to farms located within (125 miles) of Sacramento and miss many of the most heavily fertilized areas in the state,” the UC Davis researchers wrote.
Officials with the Air Resources Board couldn’t be reached for comment.
Editor's note: This article was updated April 6, 2018, to clarify that the percentage of NOx emissions from soil includes non-fertilized, natural soil.