Nitrogen source regulates soil nitrification and nitrogen losses more than nitrification inhibitor and herbicide: A laboratory evaluation

Increased environmental nitrogen (N) losses have prompted the use of enhanced efficiency nitrogen fertilizers, including nitrification inhibitors. However, the comparative effects of nitrification inhibitors with conventional nitrogen fertilizers and herbicides on soil nitrification and nitrogen losses remain poorly understood. This study evaluated the impact of a nitrification inhibitor (Instinct NXTGEN), two nitrogen sources (broadcast urea vs. injected aqueous ammonia), and a preemergence herbicide (Acuron) on (1) soil nitrification through a 25-day soil incubation experiment and (2) NH₃ volatilization, NO₃-N leaching, and N₂O-N emissions through a 31-day soil column study in loamy sand soil. In both experiments, treatments included combinations of nitrification inhibitor versus no inhibitor, two nitrogen sources, and preemergence herbicide versus no herbicide. Results revealed that nitrogen source significantly influenced nitrification, with injected aqueous ammonia reducing nitrification by 33% compared to surface broadcast urea. Nitrification inhibitors and herbicide had no effect on soil nitrification. Injected aqueous ammonia reduced NH₃ volatilization by 87% compared to surface broadcast urea, but the effect of the nitrification inhibitor on NH₃ volatilization was inconsistent across both nitrogen sources. Injected aqueous ammonia led to 39% higher cumulative nitrogen (NO₃-N + NH₄-N) leaching than urea, while the nitrification inhibitor had an inconsistent effect on NO₃-N leaching across both nitrogen sources. No significant differences in N₂O-N emissions were observed among treatments, and the herbicide had no effect on any measured parameters. These findings suggest that nitrogen source plays a more critical role in regulating soil nitrogen losses than nitrification inhibitors or herbicides.