Where Did Air Come From

Ammonia (NH ) is utilized by organisms, and it evenmaUy turns back to nitrogen (N ) or nitrate (NOj ). Nitrate is also used by organisms. However, the quantity that undergoes these reactions is miniscule compared with the entire nitrogen (N ) present in the atmosphere, which is 4x 10 kg. The amount of nitrogen biologically fixed is estimated to be 2 x 10 kg/year. 

Reactions involved in photosynthesis are very complicated, and of multiple steps, but oxygen comes from the decomposition of water. So plants produce oxygen, which animals consume, and a steady state has been established. And so, the current 21% figure (oxygen in the air) seems to be more or less constant (see also Chap. 3). 

---