Field Measurements of Nitrous Oxide Emissions From Two Cotton Sites.

The modeled effects of increasing atmospheric greenhouse gases (GHGs) on potential climate change are being widely debated. Of the three biogenic GHGs (i.e., carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O)), N₂O is considered to be the most potent. It has been estimated that in California, agricultural soils accounts for 64% of the total N₂O emissions. California’s San Joaquin Valley (SJV) is among the major producers of cotton in the United States. The overall goal of this study was to determine detailed time series measurement of N₂O fluxes at crucial management events such irrigation, fertilization, and cultivation, for two cotton sites in the Central Valley of California. The objective for site I was to determine N₂O fluxes for cotton fertilized with Urea Ammonium Nitrate (UAN 32) applied with and without a nitrogenase inhibitor. Flux chamber measurements were conducted on beds at four times during the summer. For Site II, the objective was to compare N₂O fluxes from cotton furrows and beds fertilized with Urea Ammonium Nitrate (UAN 32). The flux chamber measurements were conducted using an Environmental Protection Agency (EPA) approved methodology to collect air samples (ppm data) which were ultimately analyzed using a Gas Chromatograph (G.C.). Preliminary findings from Site I, indicated that N₂O emissions were influenced by N fertilizer rates and irrigation events. For example, field measurements of N₂O fluxes ranged from less than 10 to 40 ug N/m²/h for plots receiving 50 to 100lbs N/acre, respectively. After an irrigation event, these fluxes ranged from 20 to 80 ugN/m²/h. More importantly, the nitrogenase inhibitors reduced N₂O fluxes by as much as 50%. For Site II, N₂O fluxes from beds averaged 128 ugN/m²/d, which was approximately 31% more than that detected from the furrows.