Knife-injected anhydrous ammonia increases yield-scaled N2O emissions compared to broadcast or band-applied ammonium sulfate in wheat

The effect of nitrogen (N) application rate on nitrous oxide (N2O) emissions in grain cropping systems has been extensively studied, but little attention has been given to N source as a factor influencing emissions. To expand our understanding of the conditions that affect N2O emissions and production pathways, and develop management practices that farmers can use to maintain crop productivity while mitigating N2O emissions, we conducted field experiments during two years in wheat (Triticum aestivum) cropping systems in the Sacramento Valley, California. Five levels of total N were applied, ranging from 0 to 254 in the 2009-10 season, and from 0 to 266 kg N ha−1 in the 2010-11 season. At planting in November, either ammonium sulfate (AS) or anhydrous ammonia (AA) was applied, and either urea or calcium nitrate was used as a topdress at the stem elongation stage. The cumulative N2O emissions ranged from 0.24 (±0.07) to 1.31 (±0.35) kg N2O-N ha−1 in 2009-10 and from 0.74 (±0.24) to 2.2 (±0.23) kg N2O-N ha−1 in 2010-11. In both seasons, the highest total N2O and yield-scaled N2O emissions were recorded in the treatments that included 112 kg N ha−1 as knife-injected AA. In 2010-11, the total N2O and yield-scaled N2O emissions following 168 kg N ha−1 broadcast AS starter were similar as those of the AA treatment. Significantly higher N2O fluxes after a topdress application of 98 kg N ha−1 urea compared to a nitrate addition of the same magnitude indicates that ammonia oxidation was likely the source of N2O production. In contrast to other studies, N use efficiency was not negatively correlated to N uptake-scaled N2O emissions, mainly because N2O emissions were a function of fertilizer type and application method.