The California Global Warming Solutions Act of 2006 (AB 32) mandates that the State develop comprehensive strategies to reduce greenhouse gas (GHG) emissions. Nitrous oxide (N₂O) is a potent GHG emitted from intensively managed agro-ecosystems as a result of fertilizer N applications and leguminous cover crops. More than half the GHG emissions from agriculture are attributed to N₂O production. Currently, there is a lack of baseline N₂O emission data for most cropping systems in the State, and the relationship between N fertilization levels and N₂O emission is not well understood 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. The objectives of this project are to (1) measure annual N₂O emissions for major California crops (vineyards, almonds, tomato, wheat, alfalfa, lettuce, rice) under typical management practices,  (2) determine N₂O emission factors for these crop systems at several fertilizer N addition rates, (3) to characterize the effects of environmental factors on the temporal profile of N₂O emissions, and (4) to determine NO and NO₂ fluxes in these crop systems. 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 y^-1 at fertilizer rates of 225 and 300 kg N /ha.  About half the annual emissions were emitted within 3 d after the first seasonal rainfall event. In other tomato studies, estimated losses of fertilizer N as N₂O were 0.38 ± 0.03 kg/ha y^-1 in a drip irrigated system and 1.79 ± 0.21 kg/ha y^-1 in furrow irrigated system, which was equivalent to 0.19% and 0.73% of the added fertilizer, respectively. In a lettuce production system, annual N₂O emissions were about 1 kg N₂O-N /ha y^-1. In a wheat system, emissions during the growing season in the rainy season were between 1.0 and 1.5 kg N₂O-N, with highest emissions occurring after anhydrous ammonium applications. Annual emissions in a vineyard totaled 2.05 kg N₂O-N/ha in the first year, when a leguminous cover crop was planted in the tractor row, while emissions reached only 0.17 kg N2O-N/ha in the second year when the tractor rows were fallow. All emissions were generally lower than 1% of the applied N.