Assessing GHG In California Cropping Systems to Address Climate Change Regulation.

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.