@shortstories even in something like a spaceship with a closed loop, nitrogen would degrade into atmospheric nitrogen (N₂) which has a far lower energy state than in nitrogen compounds (which is why Nitrogen based explsovies work: high energy state -> low energy state = energy)

it takes an enormous amount of energy to break atmospheric nitrogen into ammonia industrially, and before humans could do that we simply had a hard cap on how many people could be fed by nitrogen fixing bacteria living in plant roots and lightning strikes