Investigating the Impact of Soil NO x on Air Quality in Rural Agricultural Communities: A Call to Action for Environmental Justice in California

Abstract

Disadvantaged communities in rural agricultural areas throughout the San Joaquin Valley (SJVAB) and Salton Sea Air Basins (SSAB) disproportionately bear the impacts of poor air quality, despite years of regulatory efforts to reduce pollution from combustion sources. Evidence shows that there is an overlooked, unregulated pollution source affecting these areas. The influence of soil NO_x emissions on air pollution in agricultural regions has come under scrutiny in recent years and is a potential candidate as an underestimated pollution source. However, despite many public comments and letters emphasizing the critical nature of this issue, little has been done toward investigating and regulating this putative source of air pollution. Here, we present a policy brief in representation of underserved communities in the SJVAB and SSAB of California, providing evidence from peer-reviewed research that supports this claim and offering regulatory recommendations to mitigate the overlooked source. We believe that through activating community organizations, farmers, policymakers, and scientists, we may work together to find solutions that will result in bringing these rural (and adjacent urban) areas into compliance with the Clean Air Act.

Keywords

environmental equity public policy Clean Air Act environmental regulations pollution prevention

PROBLEM

The San Joaquin Valley (SJVAB) and Salton Sea Air Basins (SSAB) are two of the most polluted air basins in the country, renowned for their immense agricultural productivity. These regions are nonattainment for three federally regulated air pollutants: ozone (O₃), fine particulate matter (PM_2.5), and coarse particulate matter (PM₁₀). Both have failed to meet O₃ National Ambient Air Quality Standards (NAAQS) throughout the history of regulation.¹ In addition, PM_2.5 concentrations in the SJVAB are so severe that the Environmental Protection Agency (EPA) has rejected the State Implementation Plans for attainment of the 2006 and 2012 PM_2.5 NAAQS. In fact, the 1997 PM_2.5 standard was only attained in the SJVAB in 2022.² Figure 1 shows the plateau behavior of O₃ and PM_2.5 design values3 in the SJVAB and the SSAB. The absence of a decreasing trend in the last 6 years for both strongly indicates a potential missing source in extant air emissions inventories, i.e., one that is outside of regulatory control.

FIG. 1.

Trends in maximum daily 8-hour (MDA8) ozone (A) and annual PM2.5 design value (B) concentrations in the San Joaquin Valley and Salton Sea Air Basins over the 21st century. These trends are compared to the urban South Coast and San Diego Air Basins. Data from California Air Resources Board (CARB)’s Air Quality Data Statistics Trends Summary website: https://www.arb.ca.gov/adam/trends/trends1.php.

Persistent exposure to some air pollutants can lead to reductions in crop yield,⁴ in addition to a wide array of impacts on human mortality and morbidity involving cardiovascular and lung disease as well as adverse birth outcomes, neurodevelopment and cognitive function, and even diabetes.⁵ Exposure to air pollutants, such as O₃ and PM_2.5, inflicts significant and irreparable harm to people, especially to those with preexisting respiratory and cardiovascular diseases, the elderly, and children.^6,7,8 In fact, Imperial County (in SSAB) held both the highest infection and mortality rates per capita in the state from COVID-19 throughout the height of the pandemic, believed to be influenced, at least in part, by these air quality issues.^9,10,11,12 In addition to air quality, these communities face a number of other injustices that affect their overall well-being.¹³ Residents of the SJVAB and SSAB are majority people of color (>70%), and 40% of residents live below the poverty line.¹⁴ Moreover, these communities face insufficient housing availability, poor infrastructure, language barriers, high rates of asthma, and impaired water quality.^15,16,17,18 Figure 2 is from CalEnviroScreen,¹⁹ an environmental justice tracker that ranks census tracts in California based on potential exposures to pollutants, adverse environmental conditions, socioeconomic factors, and prevalence of certain health conditions. It is apparent that communities in these regions rank much higher on these common environmental injustice metrics in comparison to most of the rest of the state. Environmental health and justice organizations contend with the entrenched economic power of the agricultural industry, oil and gas, transportation, and land development industries in the state. In many cases, industry power takes precedence over community health.

FIG. 2.

The environmental justice tracker (CalEnviroScreen) for the (A) Salton Sea Air Basin and (B) San Joaquin Valley Air Basin are shown.

Nitrogen oxides (NO_x = NO + NO₂) serve as important precursors to O₃ and PM_2.5, and primary NO_x sources are generated by high-temperature combustion in air from vehicles, power plants, lightning, and biomass burning.^20,21,22 However, microbial emissions of nitric oxide (NO) from nitrification/denitrification processes in soils are known to be significant, especially from fertilizer in industrial agricultural lands.^23,24,25 The production of NO in soils is a complicated process dependent on many variables, but most importantly on nitrifying and denitrifying bacteria, nitrogen and oxygen availability, soil moisture and pH, and temperature.^26,27 NO is a byproduct of both nitrification and denitrification soil processes. Nitrogen availability in agricultural soils is directly dependent on N fertilizer inputs. Thereby, the overapplication of N fertilizer results in the production of NO (as well as nitrous acid, HONO, and nitrous oxide, N₂O). HONO rapidly photolyzes into NO_x, while N₂O is a potent greenhouse gas (298 times more potent than carbon dioxide).

As traditional anthropogenic emissions from fossil fuel combustion continue to decline due to air quality (AQ) and climate regulatory efforts, largely due to the federal Clean Air Act and California’s Global Warming Solutions Act (AB32), the proportional impact of anthropogenic soil emissions is steadily increasing. Despite the long-term progress in NO_x reductions, nonattainment of national AQ standards for O₃ and PM_2.5 persists throughout intensive agricultural areas in California, like the SJVAB and SSAB.²⁸ This issue is a matter of environmental injustice because the impacts are felt most acutely in areas of high social vulnerability,²⁹ and these conditions may be avoidable with scientifically informed political action.

FINDINGS

Several independent studies support the hypothesis that soil NO_x emissions are underestimated by regulatory AQ models.^{30,31,32,33,34,35,36,37,38} These studies are summarized in Table 1, outlining key findings, including emphasis on the continual nonattainment of AQ standards in the SJVAB and SSAB, the impact of soil NO_x on AQ nonattainment, and various environmental factors and agricultural practices that may influence soil NO_x production. However, one study by CARB researchers, using a soil biogeochemical model (DeNitrification-DeComposition) that has not been extensively tested in air quality studies (that is, validated by observations of NO₂ by satellite or surface network NO_x/O₃), found soil NO_x emissions from agricultural systems across California to be negligible. Based on a study by Wang et al. (2021),³³ this underestimate is likely to be, at least in part, due to their use of a default temperature coefficient for nitrification based on temperate meteorological conditions that may not accurately reflect the influence of extreme temperatures, like those observed in the SJVAB and SSAB. This negligible (∼1%) contribution from agricultural soils in California is conceptually hard to justify given that the global average of soil sources of NO_x is on average ∼20%,³⁹ similar to the contribution from the continental US (∼17% according to Silvern et al., 2019).⁴⁰ On the contrary, 10 other studies, based on thoroughly tested models and observations, found emissions 2–50 times larger across California agricultural landscapes. The collective evidence strongly suggests that agricultural soil NO_x significantly aggravates AQ problems in rural California communities (and inland cities), making it urgent that policymakers and regulatory bodies consider these insights for effective AQ management strategies. There is a pressing need for continued research to refine simulated soil NO_x parameterizations and improve our understanding of the exact physio-chemical controls, which should emphasize collaborations between researchers, regulatory agencies, and local communities for comprehensive and sustainable solutions.

Table 1.

A Summary of Recent Literature That Addresses Agricultural Soil NO_x and Its Impact on Air Quality in California, Especially in the San Joaquin Valley and Salton Sea Air Basins

Study	Key findings
Oikawa et al. (2015)²⁴	– Reported record soil NO_x emissions (up to 280 kg N ha⁻¹ yr⁻¹) in Holtville, CA (Imperial Valley). – Default soil NO_x parameterization in Weather Research & Forecasting (WRF)-Chem model underestimates emissions by at least one order of magnitude. – Augmenting the default soil NO_x emissions (by factors of 10–64 to improve agreements with observations) increases modeled O₃ concentrations by 2–8.5 ppb throughout Imperial Valley in the SSAB.
Parrish et al. (2017)²⁷	– Observed dramatically curtailed O₃ improvements in the San Joaquin Valley and Salton Sea air basins, despite improvements in surrounding Southern California air basins. – Suggested that recalcitrant O₃ levels are linked to sources evading regulatory efforts most probably related to the intensive agriculture that characterizes these basins.
Almaraz et al. (2018)	– Suggested in 2018 that agricultural soils emit 20%–32% on average of the state’s total NO_x, with the average cropland NO_x flux around 20 kg N ha⁻¹ yr⁻¹.
Trousdell et al. (2019)³⁰	– Top-down aircraft measurements of NO_x and O₃, estimated a total emission rate of 215 ± 33 tons/day around Fresno in the SJVAB, a factor of 2 larger than the CARB’s California Emissions Projection Analysis Model (CEPAM) inventory – Rural SJV counties exhibited no observed O₃ decreases from 2006–2016.
Kleeman et al. (2019)³¹	– Modified soil emissions in their model using soil NO_x emissions from Almaraz et al. [2018]. – These modifications increased predicted PM_2.5 nitrate concentrations in January of three different years in the SJV, helping to correct a consistent underprediction by the model.
Guo et al. (2020)³²	– California Air Resources Board study using a soil biogeochemical model (DeNitrification-DeComposition, DNDC) that is mostly untested against agricultural soil NO_x emissions. – Concluded that soil emissions contribute only 1.1% of total anthropogenic NO_x in CA, although larger than normal emissions were estimated for the Imperial Valley. – Suggested default temperature coefficient for nitrification may need revision to account for hot agroecosystems like SJVAB and SSAB.
Sha et al. (2021)³⁴	– University of Iowa modified soil NO_x parameterization in WRF-Chem (Berkeley Dalhousie Iowa Soil NO Parameterization), e.g., enhanced land cover, soil temperature, emission pulses, and N fertilizer influences. – In July 2018, estimated soil emissions contributed ∼40% of California’s total NO_x, increasing rural O₃ concentrations by +23%.
Wang et al. (2021)³³	– Used a stronger, observation-based temperature response for soil NO_x. – Improved correlation between satellite NO₂ and modeled column NO₂ over the central US in the summer, suggesting soil emissions could be larger than expected at temperatures >30°C – Emphasized hotspots in the SJVAB and SSAB due to N fertilization and warm climates.
Luo et al. (2022)³⁵	– Simulated reactive N emissions across the contiguous US by county, estimating annual emissions from the eight counties of the SJVAB to total about 100 tons/day, compared with CARB’s CEPAM inventory of ∼230 tons/day anthropogenic NO_x estimate (comparable deviation to that reported in Trousdell et al. [2019]). – Incorporated daily fertilizer application estimates from the Environmental Policy Integrated Climate (EPIC) agricultural model, based entirely on simulated idealized plant demands
Wang et al. (2023)³⁶	– Ozone Monitoring Instrument (OMI) satellite NO₂ data over croplands revealed a soil-like temperature and soil moisture dependence, with no long-term trend over croplands as opposed to continued decreases (−3.7 ± 0.3 %yr⁻¹) over urban areas. – Illustrates a dependence of observed NO2 columns on temperature and soil moisture similar to that of soil emissions over a variety of non-urban land types across California, implying that soil NO_x is a significant source.
Lieb et al. (2024)³⁷	– A mixing model using CARB’s CEPAM NO_x inventory and source-dependent δ¹⁵N-NO_x isotopic measurements revealed an underestimated soil contribution. – Our adjustments showed soil NO_x to be 30% of total NO_x in the SSAB, an order of magnitude larger than CEPAM’s 3%.

INTERESTED GROUPS OF CONCERN

Key groups include community organizations in the SSAB and SJVAB, farmers, and various AQ management bodies. Community organizations will serve as stewards for community needs, while farmers and regulatory management bodies should work to ensure safe, sustainable, and efficient agricultural practices. Ensuring awareness about the health impacts of current farming practices among community members and organizations in agriculturally productive regions is crucial. Farmers should be provided alternative strategies to improve fertilizer use efficiency, ultimately reducing expenses and minimizing environmental consequences.

The responsibility lies with local, state, and federal environmental protection agencies (i.e., Imperial County Air Pollution Control District, South Coast Air Quality Management District, SJV Air Pollution Control District, CARB, State Water Resources Control Board, California Department of Food and Agriculture [CDFA], US Department of Agriculture, and the US EPA) to support comprehensive research and disseminate vital information to communities and farmers. Specifically, environmental management bodies such as CARB, CDFA, USDA, and the US EPA should be responsible for reviewing existing research and planning new projects to better understand the formation of NO (and HONO) from agricultural soils and how fertilizer, temperature, and soil moisture influence their production. These bodies are also responsible for enforcing new agricultural practices that result from research projects. Regional air quality management districts should work with state and federal agencies to ensure that their air quality complies with standards. Further, the Irrigated Land Regulatory Program (ILRP) of California requires farms to report their N fertilizer usage. This information should be shared with researchers to better understand the influence of N fertilizer usage (especially when used in excess) on the production of soil NO.

POLICY RECOMMENDATIONS

Policymakers have an important role in the reduction of soil NO_x production in agricultural communities. It is important that communities are educated about the air quality issues and concerns. Specifically, community awareness about seasonal trends influencing pollution extremes and interpreting air monitoring data for O₃ and PM_2.5 is vital for mitigating health implications. Discussing this information in public engagement forums such as AB-617 Community Air Protection Program,⁴¹ Imperial Valley Environmental Justice Task Force meetings, CARB’s Subject Matter Expert Review Panel, and other local AQ or agriculture-related meetings is imperative. Specifically, educating community members on conditions that lead to elevated O₃ and PM_2.5 concentrations should be reiterated seasonally. In addition, CCV and CARB have gone to extensive efforts to implement the Identifying Violations Affecting Neighborhoods (IVAN) air monitoring network to educate the public about their particulate matter pollution problems. This network has been expanded to other regions of the state; however, maintenance of these monitors is lacking as many of them are offline. We recommend that this network continue its expansion into more regions of the SJV and that more resources go toward maintaining the network. The IVAN network allows community members to report air quality incidents, which empowers communities to have more control over air quality issues and resolutions.

To enhance AQ management, local, state, and federal organizations should collaborate with scientists to comprehend the agricultural impact of soil NO_x in rural areas, as well as research potential control measures that can be adopted into state Clean Air Act implementation plans. This is only going to become more imperative as the US EPA implements its most recent health-based refinement of the NAAQS for annual PM_2.5 from 12 to 9 µg/m³.⁴² The new standard is going to push most of rural California out of attainment and will require additional resources to achieve compliance, specifically addressing this agricultural source of NO_x. Strengthening relationships with community organizations is key, and leveraging resources such as the EPA’s Environmental and Climate Justice Community Change Grants program could aid in addressing the soil NO_x issue. These collaborations should aim to reduce pollution, enhance climate resilience, and build community capacity to address environmental and climate justice challenges. In addition, a deeper understanding of the primary components that produce soil NO_x is crucial for assessing the manageability of soil NO_x emissions. We strongly encourage regulatory agencies and state/federal funding to allocate more resources towards research projects focused on these topics for more informed policy recommendations.

We provide here scientific evidence supporting calls for investigating the impact of agricultural soil NO_x emissions in state and federal models, along with suggesting practices to minimize emissions, while benefiting farmers, community members, and the environment. Various farming methods can be employed to reduce NO_x emissions, which most often correspondingly reduce N₂O emissions. Modern agriculture is driven by the maximization of output, which is often short-term focused, results in excessive amounts of food waste, and lacks regard for input efficiency.⁴³ Utilization of synthetic fertilizer maximizes output; however, input efficiency of N fertilizer is one of the lowest among the plant nutrients, contributing substantially to environmental degradation.⁴⁴ Furthermore, these suggestions could serve as a model for other polluted areas that suffer from soil NO_x emissions.

Management suggestions

Promote precision techniques for agricultural sustainability.

Nutrient uptake efficiency can be improved by using precision fertilization, applying fertilizer below the surface closer to the site of root uptake, as well as timing the applications to coincide with crop developmental stages.^45,46,47 Farmers should be encouraged to avoid applications of N when soil is bare and on especially hot summer days.⁴⁸

Alternative fertigation practices may reduce NO emissions.

Practices like Pump and Fertilize (P&F) and high frequency low concentration (HFLC) have been shown to reduce N₂O emissions by 52%–72% and 48%–58%, respectively, although their impacts on NO emissions need further investigation.⁴⁹ To summarize, P&F reduces the applied N rate in response to measured concentrations of N in the groundwater so that added N and groundwater N reach the same total N applied, while HFLC applies smaller N rates per fertilization event.

Biogenic methods, such as nitrogen-fixing plants, biological nitrogen inhibitors, and cover crops, reduce N outputs in soils.

Biological nitrogen-fixing plants, such as legumes, efficiently fix N₂ from the atmosphere, making N readily available in the soil.⁵⁰ These plants can act as a supplement to synthetic fertilizer, offering an economically attractive and environmentally friendly means of reducing overapplication of nutrient inputs; however, further study on their reliability is necessary. In addition, biological nitrification inhibitors (BNIs) are compounds released by plant roots of certain species (e.g., Brachiaria humidicola, sorghum) that suppress the activity of soil microorganisms responsible for nitrification.⁵¹ BNIs slow the conversion of ammonium to nitrate, decreasing the availability of N for processes that lead to the production of NO_x and N₂O, while increasing the availability for N uptake by plants. Moreover, planting cover crops improves soil structure and health while reducing N loss from soils without reducing crop yield; however, the specific impact that cover crops have on the reduction of NO emissions has not been extensively studied.⁵²

Soil remediation practices can reduce soil NO production.

Promoting the reduction of NO_x to environmentally benign gases, such as N₂, can be achieved through the remediation of riparian zones that collect runoff.⁵³ In addition, soil carbon and reduction of N and P leaching can be improved using biochar inputs.⁵⁴ The large-scale dissemination of such farming practices could ultimately reduce environmental hazards in water and air and lower farming costs. It is important to note that the amount these practices will reduce soil NO_x production is understudied; therefore, more research is necessary to determine which procedures should be implemented through policy.

Reported N inputs should be publicly available.

The ILRP, part of the California Water Resources Control Board, regulates the use of nitrogen on irrigated lands through a Total Nitrogen Applied (TNA) report to protect ground and surface waters.^55,56,57 This information is currently not publicly available to protect farmers; however, it should be accessible upon request for researchers aiming to better understand the influence of excess N fertilizer inputs on the production of soil NO_x emissions.

Cropland repurposing can help improve air quality and promote economic prosperity in disadvantaged agricultural communities.

California’s Sustainable Groundwater Management Act has stimulated discussion of land repurposing through retiring cropland. Land repurposing, especially in buffer zones, protects local rural frontline communities’ groundwater resources from agricultural overextraction, reduces pesticide exposure, and lessens the harmful air quality impacts of particulate matter.⁵⁸ In addition, repurposing retired croplands will reduce N fertilizer inputs and may even reduce legacy nitrogen depending on the management practice (e.g., reintroduction of native plants). The Environmental Defense Fund has outlined in detail strategies for repurposing retired agricultural land.⁵⁹

Areas in need of more research

Which fertilizer type and application process are best for reducing soil NO_x production and preventing other harmful air pollutants?

Oikawa et al. (2015)²⁴ showed that utilizing more complex forms of nitrogen, such as urea-based fertilizer, have been shown to reduce the rate of N diffusivity as NO.⁶⁰ Urea-based fertilizer has a higher nitrogen content (requiring less input), and is typically cheaper and less hazardous, but inefficient application may result in higher N₂O emissions, volatilization of ammonia, and increased soil acidity.⁶¹ Moreover, Slemr and Seiler (1984) showed that five times the amount of NO was emitted from soil fertilized with urea compared with ammonium nitrate; however, this study was performed on bare soil, which may not accurately reflect the state of fertilized land. N diffusivity is largely variable and depends on crop type and cycle.⁶² More research is necessary to determine which fertilizer type is most efficient at reducing the emission of N gases as well as the amount and frequency of application per crop type. In addition, more research should investigate the effectiveness of cover crops at reducing excess nitrogen in the environment.

How do environmental factors (e.g., nutrient availability, oxygen availability, temperature, soil moisture, organic carbon availability, soil structure) and regenerative agricultural practices (e.g., cover crops, soil remediation) impact the production of soil NO?

While the influence of temperature on the production of soil NO has been extensively studied, yet still debated, other environmental factors vary between field/crop type and geographic location. More extensive studies on the impacts of these factors on NO production are necessary.

AUTHORS’ CONTRIBUTIONS

Each author played a pivotal role in the development of this brief. M.R., G.A., and I.C.F. have collaborated extensively to tackle air quality challenges in the San Joaquin Valley. H.C.L. was deeply inspired by their work and her collaborative efforts with community partners Christian Torres and Luis Olmedo from Comité Cívico del Valle. Together, they sought to gain deeper insights into the agricultural impact on air quality in the Salton Sea Air Basin. I.C.F. and H.C.L. collaborated closely to construct the brief presented here, drawing from comprehensive literature reviews, research endeavors, and active engagement with community partners.

Footnotes

ACKNOWLEDGMENT

The authors would like to thank our partners at Comité Cívico del Valle, the National Parks Conservation Association, and the Central California Environmental Justice Network for their organizational and advocacy work.

AUTHOR DISCLOSURE STATEMENT

No competing financial interests exist.

FUNDING INFORMATION

This work was made possible by support from the University of California, Davis (UC Davis) Environmental Health Sciences Center (EHSC) under award number P30ES023513 of the National Institute of Environmental Health Sciences, National Institute of Health. The content is solely the responsibility of the authors and does not necessarily represent the official views of the UC Davis EHSC, nor the National Institutes of Health. Co-funding was also provided by the Western Center of Agricultural Health and Safety via NIOSH grant #U54OH007550. I.C.F.’s effort was supported by the USDA National Institute of Food and Agriculture, (Hatch project CA-D-LAW-2481-H, “Understanding Background Atmospheric Composition, Regional Emissions, and Transport Patterns Across California”). H.C.L. was supported in part by the Jastro & Shields Graduate Research Award and the Public Impact Research Initiative at UC Davis under the project titled “Helping to Improve Air Quality in Rural Communities by Raising Awareness About Unaccounted-for Emissions from Agricultural Soils.”

1

US EPA, “Nonattainment Areas for Criteria Pollutants (Green Book),” April 14, 2016, .

2

San Joaquin Valley Air Pollution Control District, “San Joaquin Valley Unified Air Pollution Control District Summary of Rules and Plans,” April 2, 2024.

3

Design value is an extreme value statistic that is approximately comparable to the values defined by the National Ambient Air Quality Standards (NAAQS). For ozone, it is the three-year running average of the 4^th highest maximum 8-hr (MDA8) ozone observed in each year. For annual PM_2.5, it is the average of 3 consecutive years of annual averages.

4

Jon Sampedro et al., “Future Impacts of Ozone Driven Damages on Agricultural Systems,” Atmospheric Environment 231 (June 2020): 117538, .

5

World Health Organization (WHO), “Review of Evidence on Health Aspects of Air Pollution – REVIHAAP Project,” 2013.

6

Fabio Caiazzo et al., “Air Pollution and Early Deaths in the United States. Part I: Quantifying the Impact of Major Sectors in 2005,” Atmospheric Environment 79 (November 2013): 198–208,

7

Aaron J. Cohen et al., “Estimates and 25-Year Trends of the Global Burden of Disease Attributable to Ambient Air Pollution: An Analysis of Data from the Global Burden of Diseases Study 2015,” The Lancet 389, no. 10082 (May 13, 2017): 1907–18,

8

Y. Meng et al., “Outdoor Air Pollution and Severe Asthma in the San Joaquin Valley, California,” Epidemiology 19, no. 6 (November 2008): S272, .

9

Gilda Zarate-Gonzalez, Paul Brown, and Ricardo Cisneros, “Costs of Air Pollution in California’s San Joaquin Valley: A Societal Perspective of the Burden of Asthma on Emergency Departments and Inpatient Care,” Journal of Asthma and Allergy Volume 17 (April 2024): 369–82,

10

Erika Garcia et al., “Long-Term Air Pollution and COVID-19 Mortality Rates in California: Findings from the Spring/Summer and Winter Surges of COVID-19,” Environmental Pollution 292 (January 2022): 118396,

11

Brittney Marian et al., “Independent Associations of Short- and Long-Term Air Pollution Exposure with COVID-19 Mortality among Californians,” Environmental Advances 9 (October 2022): 100280,

12

Marina Borro et al., “Evidence-Based Considerations Exploring Relations between SARS-CoV-2 Pandemic and Air Pollution: Involvement of PM2.5-Mediated Up-Regulation of the Viral Receptor ACE-2,” International Journal of Environmental Research and Public Health 17, no. 15 (August 2, 2020): 5573, .

13

Yaning Miao et al., “Evaluating Health Outcome Metrics and Their Connections to Air Pollution and Vulnerability in Southern California’s Coachella Valley,” Science of The Total Environment 821 (May 2022): 153255, .

14

Environmental Protection Agency, “EJScreen,” EJScreen: Environmental Justice Screening and Mapping Tool, January 23, 2024, .

15

Jonathan O. Anderson, Josef G. Thundiyil, and Andrew Stolbach, “Clearing the Air: A Review of the Effects of Particulate Matter Air Pollution on Human Health,” Journal of Medical Toxicology 8, no. 2 (June 2012): 166–75,

16

R W Atkinson et al., “Long-Term Exposure to Ambient Ozone and Mortality: A Quantitative Systematic Review and Meta-Analysis of Evidence from Cohort Studies,” BMJ Open 6, no. 2 (February 2016): e009493, ; Miao et al., “Evaluating Health Outcome Metrics and Their Connections to Air Pollution and Vulnerability in Southern California’s Coachella Valley”

17

Shohreh F. Farzan et al., “Assessment of Respiratory Health Symptoms and Asthma in Children near a Drying Saline Lake,” International Journal of Environmental Research and Public Health 16, no. 20 (October 11, 2019): 3828,

18

OEHHA, “CalEnviroScreen 4.0,” OEHHA, 2023, .

19

OEHHA, “CalEnviroScreen 4.0.”

20

Yi Tan et al., “On-Board Sensor-Based NOx Emissions from Heavy-Duty Diesel Vehicles,” Environmental Science & Technology 53, no. 9 (May 7, 2019): 5504–11,

21

U. Schumann and H. Huntrieser, “The Global Lightning-Induced Nitrogen Oxides Source,” Atmospheric Chemistry and Physics 7, no. 14 (July 24, 2007): 3823–3907,

22

Patrick C. Campbell et al., “Pronounced Increases in Nitrogen Emissions and Deposition Due to the Historic 2020 Wildfires in the Western U.S.,” Science of The Total Environment 839 (September 15, 2022): 156130, .

23

E. J. Williams and F. C. Fehsenfeld, “Measurement of Soil Nitrogen Oxide Emissions at Three North American Ecosystems,” Journal of Geophysical Research: Atmospheres 96, no. D1 (1991): 1033–42,

24

J. J. Yienger and H. Levy, “Empirical Model of Global Soil‐biogenic NO _χ Emissions,” Journal of Geophysical Research: Atmospheres 100, no. D6 (June 20, 1995): 11447–64,

25

P. Y. Oikawa et al., “Unusually High Soil Nitrogen Oxide Emissions Influence Air Quality in a High-Temperature Agricultural Region,” Nature Communications 6, no. 1 (November 10, 2015): 8753, .

26

Williams and Fehsenfeld, “Measurement of Soil Nitrogen Oxide Emissions at Three North American Ecosystems”; Yienger and Levy, “Empirical Model of Global Soil‐biogenic NO _χ Emissions”; Oikawa et al., “Unusually High Soil Nitrogen Oxide Emissions Influence Air Quality in a High-Temperature Agricultural Region”.

27

Sharon J. Hall, Pamela A. Matson, and Philip M. Roth, “NOx EMISSIONS FROM SOIL: Implications for Air Quality Modeling in Agricultural Regions,” Annual Review of Energy and the Environment 21, no. 1 (November 1996): 311–46, .

28

David D. Parrish et al., “Ozone Design Values in Southern California’s Air Basins: Temporal Evolution and U.S. Background Contribution,” Journal of Geophysical Research: Atmospheres 122, no. 20 (October 27, 2017), .

29

Ganlin Huang and Jonathan K. London, “Cumulative Environmental Vulnerability and Environmental Justice in California’s San Joaquin Valley,” International Journal of Environmental Research and Public Health 9, no. 5 (May 3, 2012): 1593–1608, .

30

Oikawa et al., “Unusually High Soil Nitrogen Oxide Emissions Influence Air Quality in a High-Temperature Agricultural Region”

31

Justin F. Trousdell et al., “Photochemical Production of Ozone and Emissions of NOx and CH4 in the San Joaquin Valley,” Atmospheric Chemistry and Physics 19, no. 16 (August 26, 2019): 10697–716,

32

Michael J Kleeman, Anikender Kumar, and Abhishek Dhiman, “Investigative Modeling of PM2.5 Episodes in the San Joaquin Valley Air Basin during Recent Years,” February 26, 2019

33

Lei Guo et al., “Assessment of Nitrogen Oxide Emissions and San Joaquin Valley PM _2.5 Impacts From Soils in California,” Journal of Geophysical Research: Atmospheres 125, no. 24 (December 27, 2020): e2020JD033304,

34

Yi Wang et al., “Improved Modelling of Soil NO _x Emissions in a High Temperature Agricultural Region: Role of Background Emissions on NO ₂ Trend over the US,” Environmental Research Letters 16, no. 8 (August 1, 2021): 084061,

35

Tong Sha et al., “Impacts of Soil NO _x Emission on O ₃ Air Quality in Rural California,” Environmental Science & Technology 55, no. 10 (May 18, 2021): 7113–22,

36

Lina Luo et al., “Integrated Modeling of U.S. Agricultural Soil Emissions of Reactive Nitrogen and Associated Impacts on Air Pollution, Health, and Climate,” Environmental Science & Technology 56, no. 13 (July 5, 2022): 9265–76,

37

Yurun Wang, Ian C Faloona, and Benjamin Z Houlton, “Satellite NO ₂ Trends Reveal Pervasive Impacts of Wildfire and Soil Emissions across California Landscapes,” Environmental Research Letters 18, no. 9 (September 1, 2023): 094032,

38

Heather C. Lieb et al., “Nitrogen Isotopes Reveal High NOx Emissions from Arid Agricultural Soils in the Salton Sea Air Basin,” Scientific Reports 14, no. 1 (November 20, 2024): 28725, .

39

Lyatt Jaeglé et al., “Global Partitioning of NOx Sources Using Satellite Observations: Relative Roles of Fossil Fuel Combustion, Biomass Burning and Soil Emissions,” Faraday Discussions 130 (2005): 407, .

40

Rachel F. Silvern et al., “Using Satellite Observations of Tropospheric NO2 Columns to Infer Long-Term Trends in US NOx Emissions: The Importance of Accounting for the Free Tropospheric NO2 Background,” Atmospheric Chemistry and Physics 19, no. 13 (July 12, 2019): 8863–78, .

41

California Air Resources Board, “Community Air Protection Program | California Air Resources Board,” 2017, .

42

US EPA, “Final Reconsideration of the National Ambient Air Quality Standards for Particulate Matter (PM),” Overviews and Factsheets, January 24, 2024, .

43

Eugene P. Odum, “Input Management of Production Systems,” Science 243, no. 4888 (1989): 177–82.

44

B.B. Bohlool et al., “Biological Nitrogen Fixation for Sustainable Agriculture: A Perspective,” Plant and Soil 141 (1992): 1–11.

45

Roland Harrison and J. Webb, “A Review of the Effect of N Fertilizer Type on Gaseous Emissions,” 2001, https://www.sciencedirect.com/science/article/pii/S0065211301730052/pdf?md5=76d7e69577cfa3eafa4a8091835f76ca&pid=1-s2.0-S0065211301730052-main.pdf

46

K. A. Smith et al., “PA—Precision Agriculture: Reduction of Ammonia Emission by Slurry Application Techniques,” Journal of Agricultural Engineering Research 77, no. 3 (November 1, 2000): 277–87,

47

U Skiba, D Fowler, and K A Smith, “Nitric Oxide Emissions from Agricultural Soils in Temperate and Tropical Climates: Sources, Controls and Mitigation Options,” Nutrient Cycling in Agroecosystems 48, no. 139–153 (1997).

48

Hall, Matson, and Roth, “NO _x EMISSIONS FROM SOIL.”

49

Patrick K. Nichols et al., “Alternative Fertilization Practices Lead to Improvements in Yield-Scaled Global Warming Potential in Almond Orchards,” Agriculture, Ecosystems & Environment 362 (March 2024): 108857, .

50

Bohlool et al., “Biological Nitrogen Fixation for Sustainable Agriculture: A Perspective.”

51

Devrim Coskun et al., “Nitrogen Transformations in Modern Agriculture and the Role of Biological Nitrification Inhibition,” Nature Plants 3, no. 6 (June 6, 2017): 17074, .

52

Skiba, Fowler, and Smith, “Nitric Oxide Emissions from Agricultural Soils in Temperate and Tropical Climates: Sources, Controls and Mitigation Options.”

53

Eric A. Davidson et al., “Excess Nitrogen in the U.S. Environment: Trends, Risks, and Solutions,” Issues in Ecology, no. 15 (2012).

54

Yingjie Dai et al., “Utilization of Biochar for the Removal of Nitrogen and Phosphorus,” Journal of Cleaner Production 257 (June 2020): 120573, .

55

California Water Boards, “Irrigated Lands Program Total Nitrogen Applied (TNA) Report Instructions,” 2024,

56

Thomas Harter, “California’s Agricultural Regions Gear up to Actively Manage Groundwater Use and Protection,” California Agriculture 69, no. 3 (July 2015): 193–201,

57

California State Water Resources Control Board, “Porter-Cologne Water Quality Control Act,” Water Code Division 7 and Related Sections, January 2024.

58

Angel Santiago Fernandez-Bou et al., “Water, Environment, and Socioeconomic Justice in California: A Multi-Benefit Cropland Repurposing Framework,” Science of The Total Environment 858 (February 2023): 159963, .

59

Environmental Defense Fund, “Advancing Strategic Land Repurposing and Groundwater Sustainability in California: A Guide for Developing Regional Strategies to Create Multiple Benefits,” 2021, .

60

Oikawa et al., “Unusually High Soil Nitrogen Oxide Emissions Influence Air Quality in a High-Temperature Agricultural Region.”

61

Oikawa et al.; Frank C. Thornton, Bert R. Bock, and Don D. Tyler, “Soil Emissions of Nitric Oxide and Nitrous Oxide from Injected Anhydrous Ammonium and Urea,” Journal of Environmental Quality 25, no. 6 (November 1996): 1378–84, .

62

F. Slemr and W. Seiler, “Field Measurements of NO and NO2 Emissions from Fertilized and Unfertilized Soils,” Journal of Atmospheric Chemistry 2, no. 1 (March 1, 1984): 1–24, ; Skiba, Fowler, and Smith, “Nitric Oxide Emissions from Agricultural Soils in Temperate and Tropical Climates: Sources, Controls and Mitigation Options.”