The field soils were also tested under 3% O2 conditions, as a representative level of oxicity for drip zones, to compare N2O production from nitrifier denitrification with heterotrophic denitrification. NO3 -based fertilizers have often been proposed for high frequency or “just-in-time” N application , and are suggested as a means to improve NUE in tree crops . Therefore a NO3-based formulation was also tested against UAN within the high-frequency system. This formulation was not tested in the standard 4x year 1 applications because the high NO3 concentrations entailed were considered to present an unacceptable leaching hazard.