Nitrogen oxide emissions from soil in the Salton Sea air basin of California are found to be 10 times higher than what the state had previously estimated, according to a new study that examined the rural Imperial and Coachella valleys.
A recent investigation by the University of California, Davis, reveals that at least 25% of nitrogen oxide (NOx) emissions in the Salton Sea air basin originate from the soil.
The study utilized isotopic analysis and discovered that the average annual total emissions from the soil in this basin are around 11 tons per day. This figure is significantly greater—10 times more—than the current state inventory for soil NOx emissions for this area. The findings have been published in the Nature journal Scientific Reports.
This study underscores the importance of understanding and accurately accounting for emissions from agricultural soils to meet state and federal air quality standards and to enhance air quality in the rural Imperial and Coachella Valleys, as well as in other agricultural regions that experience warm climates.
Underestimated sources
In the past few decades, air quality in urban areas like Los Angeles has improved significantly, mainly due to decreased fossil fuel emissions from industrial and transportation sources. However, NOx emissions seem to be increasing in several rural locations, partly due to wildfires and rising emissions from agricultural soils.
“I predict that wildfire and soil emissions will become the leading causes of air pollution this century, and we need to revise our air quality regulations in response to this, especially considering its effects on rural and suburban communities,” stated Ian Faloona, the corresponding author and a professor at UC Davis in the Department of Land, Air and Water Resources and the Air Quality Research Center. “Air quality improvement has stagnated in many areas of California over the past eight years, which I believe is largely attributed to these two underrecognized sources.”
Fertilizers, irrigation, and climate
The Salton Sea region is recognized as one of the most polluted air basins nationwide. Spanning from Palm Springs, through the Coachella Valley and Imperial Valley, and ending at the U.S.-Mexico border in Calexico, this area generates over $2 billion in agricultural sales each year with crops cultivated throughout the year. Its predominantly Latino population faces high rates of poverty, asthma, and respiratory diseases.
NOx is a crucial air pollutant that contributes to the development of both ozone and particulate matter (PM) in the atmosphere.
The significant usage of fertilizers, irrigation, and increasing temperatures are known to heighten NOx emissions from soil, especially in the studied area. California experiences some of the fastest rises in warm-season temperatures nationwide. Elevated temperatures can increase the risk of wildfires and stimulate greater microbial activity in the soil. Additionally, fertilizer sales for the sandy soils of Imperial County have surged by 137% since 1991, according to the study.
On-site sampling and laboratory analysis
To delve deeper into these factors, the researchers conducted field sampling between June 2022 and April 2023 at a high school in Imperial Valley and an air quality station on Torres Martinez Desert Cahuilla Indian land within the Coachella Valley, in collaboration with the community nonprofit Comité CÃvico del Valle.
The collected samples were transported to the UC Davis Stable Isotope Facility for extraction. The isotopic signatures were then analyzed using a Bayesian Mixing Model to determine the contribution of soil to the overall isotopic signature. This analysis revealed that soil sources of NOx emissions are currently underestimated by a factor of 10. Factors such as nutrient availability, soil moisture, and temperature influence soil NOx production.
The study’s authors also mention that gathering additional data on fertilizer application rates and irrigation practices could help clarify the direct impacts of these factors, enabling regulators to more effectively enhance air quality regulations in the area.
“Soil emissions and wildfire smoke are often dismissed as ‘background’ pollutants, but they are on the rise and are becoming significant contributors to pollution in various parts of the state,” Faloona explained. “Currently, we are failing to account for vital components of our pollution challenges.”
The research was led by Heather Lieb, who recently completed her Ph.D. under Faloona’s guidance at UC Davis.
This research received funding from the UC Davis Environmental Health Sciences Center through the National Institutes of Health, as well as from the Western Center of Agricultural Health and Safety and the USDA National Institute of Food and Agriculture.