Research Projects

Project at a Glance

Project Status: complete

Title: Determining NOx emissions from soil in California cropping systems to improve ozone modeling

Principal Investigator / Author(s): Horwath, William R.

Contractor: UC Davis

Contract Number: 09-329

Research Program Area: Emissions Monitoring & Control

Topic Areas: Agriculture, Natural (Biogenic) Sources


Soils are a source of oxides of nitrogen (NOx = nitric oxide and nitrogen dioxide), precursors for the production of ozone (O3), an air pollutant in the troposphere. Production of nitric oxide (NO) occurs through soil microbial processes using ammonium from nitrogen fertilizer and manure inputs or soil mineral nitrogen (N). Emissions of NOx were measured in almond, alfalfa, tomato, wheat, and silage corn cropping systems during summer months to obtain estimates of NOx emissions that could potentially be used in regional models predicting O3 in the San Joaquin Valley. The lowest average NOx fluxes ( < 0.1 g NOx-N ha-1 h-1) were measured at low soil moisture and in subsurface drip-irrigated tomato. The highest average emissions (0.52.8 g NOx-N ha-1 h-1) occurred in high N input systems, such as silage corn. In alfalfa, almond, and furrow-irrigated tomato, average NOx fluxes were intermediate (0.10.5 g NOx-N ha-1 h-1). The NOx emissions were related to N inputs, time since fertilizer applications, temperature, and soil moisture. Under field conditions NOx fluxes increased 2.5-3.5-fold for each increase in soil temperature of 10C. The NOx emissions seem predictable in systems receiving N at recommended rates, ranging from 0.02 2.5 g NOx-N ha-1 h-1 in alfalfa, wheat, tomato, and almond, but in systems receiving large N inputs resulting in high concentrations of ammonium, episodes of very high NOx emissions ( > 40 g NOx-N ha-1 h-1) were measured. These high NOx flux events are difficult to predict.


For questions regarding this research project, including available data and progress status, contact: Heather Choi at (916) 322-3893

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