In the early 20th century, we humans developed the technology of artificial nitrogen (N) fixation to enable industrial production of ammonia. Since then, the amount of ammonia synthesized has increased dramatically since the so-called Green Revolution in the 1960s, and the artificially created reactive N has greatly contributed to increase food production as chemical fertilizers. The created reactive N is also used for industrial materials such as chemicals, explosives, gunpowder, plastics, etc., and in recent years, the use of ammonia as a hydrogen carrier and fuel has also attracted attention. These are the benefits of N use. On the other hand, much of the N we use leaks into the environment while remaining reactive. These cause the threat of N pollution . The trade-off between the benefit of our N use and unwanted N pollution accompanied is called the N issue.