Also the energy density of liquid ammonia is only about half that of liquid hydrocarbons. In the family of nitrogen fuels there's also hydrazine, which is much spiffier, but if you're worried about the toxicity of ammonia, well.. Yeah, maybe we don't want civilians filling up their cars with hydrazine.

The benefit of nitrogen fuels as a carbon-neutral fuel is that getting nitrogen from the atmosphere is a lot easier than extracting carbon dioxide from the atmosphere.