Effects of soil amendment with biochar and irrigation on the fate of nitrogen fertilizer

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.