Annual Nitrous Oxide Emissions in Response to Nitrogen Fertilizer Rates in Lettuce, Tomato, Wheat and Alfalfa Production Systems.

Accurate estimates of nitrous oxide (N₂O) emissions from N fertilizer applications are needed to develop best management practices that minimize these emissions without decreasing yield potential. Nitrogen fertilizer was applied at five rates ranging from 0 (control) to 340 kg N ha^-1 in lettuce, tomato and wheat production systems in California, and N₂O flux is being measured by the static chamber technique for two years. In alfalfa, N₂O flux is assessed in a first- and a fourth-year stand.  The lettuce crop was sub-surface drip-, tomato was furrow-, and alfalfa was flood-irrigated, whereas wheat was rain-fed. In all treatments in lettuce, during the first year, N₂O emissions were <1 kg N ha^-1 or <0.3% of the applied N, whereas yields were similar except for the control. In lettuce grower fields, N₂O fluxes varied greatly with the highest daily emissions reaching 0.7 kg N ha^-1. In tomato, N₂O-N in the first year was <1%, and in wheat <0.5% of the applied N. In alfalfa, N₂O fluxes were not substantial. Reduction of high N₂O emissions may be possible through adjustment of N fertilization rates after accounting for residual inorganic N.