Research Projects

Project at a Glance

Project Status: complete

Title: Agricultural systems in the San Joaquin Valley: development of emissions estimates for nitrogen oxides.

Principal Investigator / Author(s): Matson, Pamela

Contractor: UC Berkeley

Contract Number: 94-732

Research Program Area: Emissions Monitoring & Control, Atmospheric Processes

Topic Areas: Agriculture, Area Sources


The objective of this study was to estimate emissions of nitrogen oxides (NOx) from agricultural systems in the San Joaquin Valley of California during the late summer period of maximum tropospheric ozone development. Nitrogen oxide fluxes were measured during July, August and early September of 1995. Field sites that were utilized for sampling represented the most important crop types and the dominant fertilizer and irrigation management practices for the area. Hourly and daily flux data along with a spatial data base of crop type areas were used to extrapolate fluxes to county and Valley scales. Soil, climatic and management factors that were important in controlling the rate and timing of NOx flux from soil were identified.

Information on crop acreage for eight San Joaquin Valley counties was used to identify nine dominant crop types including: alfalfa, citrus, corn, cotton, grapes, irrigated pasture, stonefruits, sugar beets, vegetables and other. Twenty-eight agricultural systems were identified that represented the most important crop types and the dominant fertilizer and irrigation management practices of the area. Diel measurements were carried out at least once on four sites; 13 sites were sampled repeatedly over several week periods in order to estimate variation in fluxes within sites over time.

Soil water filled pore space (WFPS), soil temperature, air temperature, soil ammonium and nitrate, total soil organic nitrogen and organic carbon, soil pH and soil texture were determined for all sites. Net and gross nitrogen mineralization and nitrification and nitrification potentials were also measured for a subset of the sites.

There was substantial variability in NOx fluxes among crops (crop mean fluxes at mid-day ranging from 1.0-9.1 ng-N cm-2h-1), with irrigated pastures, almonds and tomatoes having generally high mean fluxes relative to the other crops. Variation among different fields of the same crop type was also very large (e.g., 0.13-17.53 ng-N cm-2h-1 for cotton, 0.16-15.69 ng-N cm-2h-1 for corn) and appeared to be related to proximity in time to a fertilizer application and soil moisture characteristics. The low fluxes measured from many of the sites during the July-August period reflected the management practices for that period of time. There was relatively little application of fertilizer to crops during this mid-summer period. Sites sampled during or immediately after fertilizer application (one each of almonds, corn, and cotton) showed substantially higher NOx flux values than did the same fields or other fields of the same crop type when they were not sampled soon after fertilization.

Two types of regression models were developed to relate NOx fluxes to environmental or soil variables: 1) Point-predictive model, which is driven by information on crop type, WFPS, soil texture, soil temperature, soil NO3- and NH4+, total soil C and N and field position. 2) Management model, which is driven by crop type, fertilizer characteristics, WFPS, pH and air temperature. The management model was designed to utilize more generally available data for the development of regional emissions estimates. By incorporating important controllers of NOx flux from soil, especially WFPS and temperature, the management model developed and evaluated here improves our capacity to predict NOx fluxes under a variety of cropping and management regimes as compared to single-factor empirical models.

GIS-based data on major crop types in the San Joaquin Valley was used to calculate hourly and daily NOx flux by crop type and county, which could then be summed to estimate total flux for the Valley. Cotton, which had an intermediate mean mid-day hourly flux in comparison with other crops, had the highest total Valley hourly flux (232,120.9 g-N h-1) due to a large total acreage. Grapes were calculated to have the next largest total flux when summed over the Valley. Total flux values however can mask the spatial component of the fluxes which may be critical in determining air chemistry. Spatial distribution of NOx fluxes was presented for seven San Joaquin Valley counties. Total mid-day hourly flux ranged from 60,265 to 188,422 g-N h-1, with the counties with the highest to lowest fluxes following this sequence: Fresno, Kern, Tulare, San Joaquin, Merced, Kings, Madera.


For questions regarding research reports, contact: Heather Choi at (916) 322-3893

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