Assessment of Baseline Nitrous Oxide Emissions in California Cropping Systems

Production of the greenhouse gas (GHG) nitrous oxide (N2O) results from the activity of soil microorganisms. Nitrogen (N) inputs, soil moisture, and carbon (C) availability stimulate the production of N2O which contributes about one third of the total GHG emissions from California’s agriculture sector. Crop yields and cumulative N2O emissions were measured for two years in tomato, wheat, lettuce, and rice cropping systems fertilized at various N rates. Alfalfa was also monitored. The N2O emissions were observed to occur mainly during the growing season from soils with high moisture content after N fertilizer applications. In most cases, N2O emissions increased with increasing N rates. The levels of N application rates that resulted in the lowest N2O emissions while not reducing yields were identified. In alfalfa, a recently established one-year old stand released half as much N2O compared to a 5-year old stand. This study generated a database of field-level N2O emission measurements and accompanying soil moisture and temperature, and mineral N values that can be used to calculate the percentage of applied N released as N2O to estimate emissions on a larger scale, calibrate and validate N2O emission biogeochemical models, and develop recommended best management practices. Executive Summary Background Nitrous oxide (N2O) production in agricultural soils is a microbial process that utilizes inorganic forms of nitrogen in soil. The processes influencing the release of N2O are affected by fertilizer N inputs, soil resources such as available carbon, and soil moisture. Nitrous oxide emissions from agricultural soils contribute about one third of California agriculture’s total net greenhouse gas (GHG) emissions [California Air Resources Board (CARB), 2011]. Under the statutory authority of the California Global Warming Solutions Act of 2006 (AB32), CARB has identified research on N2O emissions from agricultural land as a priority. One of the limitations in understanding the factors affecting these N2O emissions is the dearth of information to predict N2O emission potential in the large variety of California cropping systems. Only few field level measurements of N2O emissions have been published so far, and comprehensive datasets that could be used to calibrate and validate biogeochemical models or derive cropping systemspecific emission factors (EFs) to calculate the percentage of fertilizer N emitted as N2O have been missing altogether. There is an urgent need for baseline N2O emission data to improve GHG inventories. The present study provides complete 2-year datasets of N2O emissions and related agronomic and environmental variables in important crops (tomato, wheat, alfalfa, lettuce, and rice) that represent substantial acreage and production in California. Methods Nitrous oxide emissions and soil variables were regularly measured for two years in the above cropping systems under typical management with varying N fertilizer applications. Annual N2O emissions were calculated and yields were measured to identify management practices that minimize N2O without negatively affecting crop yield potential.