New Study Finds Nitrogen Fertilizer a Major Contributor to Smog in California

Source: By Jessie Stolark, EESI • Posted: Monday, February 5, 2018

A new study led by researchers at the University of California, Davis, finds that agriculture is one of the leading cause of smog formation in the state. When nitrogen fertilizer is applied to soil, excess nitrogen is volatilized or leached out and forms nitrogen oxides (NOx), a primary component of smog. Researchers found that nitrogen application, primarily in the Central Valley, is likely contributing 25 percent or more to state-wide NOx pollution levels.  The findings are consistent with estimates of other highly productive agricultural regions around the world, but contradict the California Air Resources Board (CARB), which considers soil emissions of NOx as negligible. Additionally, the study raises questions on how best to combat NOx emissions from the agricultural sector as producers seek to intensify food production in the next several decades.

NOx is a primary component of smog and is a major cause of a myriad of health issues. It is also a potent greenhouse gas, with a global warming potential 298 times higher than carbon dioxide over a 100-year period. According to the World Health Organization, NOx is responsible for one in eight premature deaths worldwide. Both state and federal policies have focused on reducing NOx from the power and transportation sectors. Between 2005 and 2008 alone, NOx was reduced 9 percent per year in Los Angeles, San Francisco, and Sacramento, thanks to stringent air quality standards which have led to more efficient, cleaner cars and trucks.  As vehicle emissions are tightened, the relative contribution from nitrogen fertilizer to NOx will only continue to increase.

California contributes over 12 percent of the U.S. food supply, with productivity being concentrated in the Central Valley. The region produces the majority of U.S. almonds, walnuts, raisins, avocados and tomatoes.  One result of this highly productive agricultural sector is California’s rural communities are now suffering from worse air quality than their urban counterparts. At the same time, the Central Valley is home to some of the poorest communities in California. As smog-causing NOx largely remains local to its source, local reductions in NOx will have outsized benefits to local community health.

The research, published in Science Advances, is the first to estimate state-wide ag-sector NOx emissions, through a combination of real-world NOx measurements and models. The findings contradict the CARB estimate of agriculture’s contribution to state-wide NOx pollution.  CARB estimates 3.8 percent of NOx pollution is due to agriculture, compared to the study’s results of anywhere between 21 to 41 percent of NOx pollution coming from agriculture. The range is due to various potential rates of uptake of NOx from vegetation, further study would be needed to pin down the absolute contribution of agriculture to NOx pollution.

The difference between CARB’s estimate and the study, according to researchers, is due to the study’s widespread, systematic approach that also includes the highly-productive Central Valley. CARB’s sampling excludes some of the most highly productive agricultural regions in the state.

The study authors suggest both high and low-tech solutions to increase nitrogen efficiency, which reduces the amount of nitrogen applied to soils and stops its loss once it is applied.  Solutions include building soil health through returning organic content to the soil and buffer strips to reduce the loss of nitrogen to waterways.  Higher tech solutions include precision agriculture which enables producers to manage fertilizer applications in a highly-localized manner as well as irrigation management. Additionally, lost nitrogen presents an expensive problem for farmers – with estimates of lost nitrogen fertilizer at $210 billion per year in the United States alone.

Senior study author Ben Houlton, a professor at the UC Davis Department of Land, Air and Water Resources commented that technological breakthroughs are needed for the ag-sector to deal with the problem, if highly productive regions are to sustain and indeed intensify production over the next several decades. Using the example of the catalytic converter, which greatly reduced NOx from vehicle tailpipes, Houlton commented, “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.”

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