Nitrous Oxide Emissions from
Agricultural Soils in California

This page updated October 15, 2012

ARB Joint Research Seminar

Wednesday, November 28, 2012
1:30 pm, PST
Sierra Hearing Room, Second Floor
1001 "I" Street, Sacramento

This event is being Webcast, click here to view
Webcast viewers: Please send your questions during broadcast to: sierrarm@calepa.ca.gov
Presentation will be available at this link:

William Horwath, Ph.D.
Department of Land, Air, and Water Resources
University of California, Davis

Johan Six, Ph.D.
Department of Plant Sciences
University of California, Davis

Dave Goorahoo, Ph.D.
Plant Sciences Department and the Center for Irrigation Technology
California State University, Fresno

A joint seminar hosted by:
Air Resources Board (ARB)
California Energy Commission (CEC)
California Department of Food and Agriculture (CDFA)
California Department of Resources Recycling and Recovery (CalRecycle)

Nitrous oxide (N2O) is a potent greenhouse gas (GHG) derived primarily from agricultural soils receiving nitrogen fertilizers. As part of AB32 scoping plan, the Air Resources Board (ARB) identified the need for “Collaborative Research to Understand Nitrous Oxide Emissions from Nitrogen (N) Land Application” as an Early Action measure. In 2008 ARB, in coordination with three other State agencies, CEC, CDFA, and CalRecycle, initiated a nitrous oxide research program to investigate N2O emissions from major California cropping systems. This program recognized the extraordinary variability of N2O emissions from agricultural soils, and implemented a comprehensive strategy to address the complexity of N2O emissions in agricultural ecosystems using both field monitoring and geochemical modeling approaches. Since its inception, the N2O research program has funded eight projects for a total of $2.9 million from the four State agencies involved. The seminar will present results of the field studies on N2O emissions that were conducted by three research groups from UC Davis and California State University, Fresno, covering a total of nine California cropping systems: tomatoes, wheat, lettuce, alfalfa, rice, corn, cotton, almonds, and grapes. The speakers will review emission measurements of N2O, discuss the significance of these measurements, and explore challenges facing N2O emission studies in agricultural systems.

This joint seminar will offer an opportunity for the general public to interact in an open panel discussion with the investigators and provide feedback on our research program during a designated Q & A period.

N2O is a potent GHG emitted from intensively managed agro-ecosystems as a result of fertilizer N applications and leguminous crops cultivation. Currently, there is a lack of baseline N2O emission data for California cropping systems, as well as an adequate understanding of the relationship between N fertilization application rate and N2O emissions at the farming field level. The paucity of N2O emission data has hampered biogeochemical modeling, which requires robust data of N2O emissions to calibrate and validate the models’ predictive capability. We present results of field experiments in five major California cropping systems (tomato, wheat, alfalfa, lettuce, and rice) that were conducted to measure N2O emissions under typical management practices, and explore effects on emissions of different N application rates, type of fertilizers, and irrigation practices. For instance, annual emissions in processing tomato systems were 2.5 kg N2O-N/ha with standard fertilization (160 kg N/ha), similar to background emissions and those from a drip irrigated system, and 4.0 and 5.8 kg N2O-N ha-1 at fertilizer rates of 225 and 300 kg N ha-1.  About half the annual emissions were emitted within 3 days after the first seasonal rainfall event. In addition, discussions on emissions of NOx from three California dairy systems are presented.

In California agriculture is responsible for 6% of the total GHG emissions, with almost 40% as N2O. Over the past several years, growers and researchers have teamed up to develop economically feasible practices to mitigate GHG emissions within California agriculture. One of the main challenges before effective mitigation practices can be developed, is obtaining accurate estimates of current GHG emissions due to the high temporal and spatial variation, especially of N2O, across crop types, soil types, and climates. We measured background N2O emissions from perennial cropping systems (vineyards and almond orchards); compared conventional versus integrated management in tomato crops; and tested the effect of biochar amendments on N2O emissions within vegetable crops and walnut orchards. Vineyard N2O emissions without and with a leguminous cover crop ranged from 0.2 to 2.0 kg N2O-N ha-1 yr-1, respectively, while the almond orchard averaged 0.4 kg N2O-N ha-1 yr-1. The conventional tomato cropping system yielded 2.1 kg N2O-N ha-1 yr-1 while the integrated system less than half that, with only 0.9 kg N2O-N ha-1 yr-1. Results from a walnut biochar study show slight increases in N2O emissions compared to the untreated control, however, reduced emissions were found when the same biochar was applied to lettuce crops.

The overall goals of our current research are to: (1) determine detailed time series of N2O fluxes and underlying factors at crucial management events (e.g. irrigation, fertilization, and cultivation) in representative agro-ecosystems in the San Joaquin Valley (SJV) of California; and (2) use the intensive data on N2O fluxes to calibrate and validate the processed based biogeochemical De-Nitrification - De-Composition model (DNDC). Specific objective of this phase of the project funded by CDFA is to determine N2O flux measurements for cotton, silage corn and vegetable cropping systems in the SJV.  Given the interest in the suitability of current emission factors for estimating N2O emissions, we are attempting to determine the percentage of N lost to the atmosphere as N2O from added N fertilizer for cotton, silage corn and tomatoes. We review the methodology and challenges associated with data collection and analyses in a system’s approach that considers N fertilization, crop N use, N loss as N2O, and the soil physical and chemical environment. Sampling techniques and current status of a sampling plan for intensive measurements of N2O fluxes during periods with high N2O emission potential, and less frequent monitoring of N2O emissions when fluxes are low, for baseline and event related N2O emissions will also be discussed.

William Horwarth, Ph.D., is a professor of soil biogeochemistry in the Department of Land, Air and Water Resources at the University of California, Davis. Dr. Horwath is a recognized leader in sustainable agriculture research. Dr. Horwarth's research has aimed to understand nutrient limitations to crop production and form the basis of understanding the sustainability of agriculture. Professor Horwarth's work is wide ranging, encompassing determining microbes as precursors of soil humic substances, effect of agricultural practices on water quality and greenhouse gas emissions, enzyme control of nitrogen uptake by microorganisms, etc. Dr. Horwath was appointed the James G. Boswell Endowed Chair in Soil Science in 2008 and was elected as a fellow of the Soil Science Society of America in 2009.

Johan Six, Ph.D., is a professor in agroecology in the Department of Plant Sciences at the University of California, Davis. Dr. Six has been involved and led many projects investigating the effect of land use change and management on greenhouse gas fluxes in agricultural, grassland and forest ecosystems. Professor Six conducts experimental work at both the plot and landscape levels and subsequently integrates it with simulation modeling to identify gaps in our knowledge, generate testable hypotheses, underpin the mechanistic bases of the models, and predict ecosystem responses to global change at the field, landscape and regional scale. Recently, bioeconomic modeling has become a major component of his research in order to address policies for sustainable agriculture from both a biophysical and economic standpoint. Dr. Six is a Chancellor’s Fellow of the University of California, Davis and a Fellow of the American Association for the Advancement of Science. Dr. Six serves on several Science Advisory groups of national and international governmental and non-governmental organizations dealing with Climate and Agriculture.

Dave Goorahoo, Ph.D., is an associate professor in the Plant Science Department and a soil scientist with the Center for Irrigation Technology (CIT) at California State University, Fresno.  Dr. Goorahoo's research focuses on nutrient and water use efficiency in vegetable crop production systems with an emphasis on examining the impact of agricultural practices on our air, soil and water resources. In particular Dr. Goorahoo has been involved in Nitrogen (N) research aimed at quantifying various components of N budgets in an effort to estimate nitrate (NO3) leaching and gaseous ammonia (NH3) and nitrous oxide (N2O) emissions. In 2008 Dr. Goorahoo received the Coleman Fellowship for “Entrepreneurship in Organic Farming”, and was the winner of the JCAST Outstanding Research & Scholarly Activity (ORSA) Award. In 2011 and 2012 Dr. Goorahoo was nominated to the Campus Advisers Network (CAN) for the outstanding advisor award.

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For information on this Seminar please contact:
Lei Guo, Ph.D., at (916) 322-8097 or send email to: lguo@arb.ca.gov

For information on this Series please contact:
Peter Mathews at (916) 323-8711 or send email to: pmathews@arb.ca.gov

For a complete listing of the ARB Research Seminars and the related documentation
for the seminars please check this page

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