Several agricultural regions in California are in the process of establishing regulations to mitigate the statewide groundwater nitrate (NO3-) contamination problems. Biochar has the potential to reduce N leaching and/or improve N availability in soil for plant uptake because of its strong adsorption of ammonium (NH4+) and the possibility of delaying nitrification process that were determined in laboratory experiments. A field study was conducted to determine the effects of biochar amended soil on N dynamics during an onion growing season. Treatments included amendments of biochar and biochar plus manure under two irrigation levels (100% and 75% of water supply to maintain a reference of soil water content). Surface drip irrigation and fertigation were applied to raised-beds. Ammonia (NH3) and nitrous oxide (N2O) emissions, and soil pore water N (>95% as NO3-) concentrations below the rooting zone were determined during the growing season. Fertilization and irrigation levels appeared to significantly affect NH3 loss. Over time, soil pore water N concentration became much higher at 50 cm soil depth than that at 25 cm depth indicating a high leaching risk from both irrigation levels. Plant N uptake is being monitored. All data will be summarized and presented to illustrate the challenges in N management for irrigated vegetable production and potential use of biochar to develop strategies to reduce N loss to the environment.