Nitrogen (N2) gas is the most abundant element in Earth's atmosphere-- approximately 78%, but is nearly unusable in this elemental form. Nitrogen as molecular component is an essential ingredient in both plant and animal proteins, but is typically the limiting factor in plant growth. Nitrogen fixation is done in nature either by lightning or by diazotrophic bacteria using an enzyme called nitrogenase as a catalyst. Natural nitrogen fixing mechanisms are quickly being surpassed by human industrial production through the Haber-Bosch process. Some of the nitrogen compounds are converted by plant eating animals into proteins but mostly it is processed as waste, in the form of ammonia in urine. Ammonia is converted by other bacteria back into nitrites, nitrates, and elemental N2 gas.
If an adequate amount of previously fixed nitrogen is included at the the startup of a CELSS, it is conceivable that the lower loop may not be necessary to the operation of the CELSS. The main purposes of including unfixed nitrogen gas as an atmospheric component are as follows: 1. It is easy to obtain from Earth's atmosphere and difficult to remove. 2. To increase total gas pressure (to equalize with outside pressure without increasing partial pressure of reactive oxygen) 3. It can provide a buffer to the nitrate supply.
If an adequate amount of previously fixed nitrogen is included at the the startup of a CELSS, it is conceivable that the lower loop may not be necessary to the operation of the CELSS. The main purposes of including unfixed nitrogen gas as an atmospheric component are as follows: 1. It is easy to obtain from Earth's atmosphere and difficult to remove. 2. To increase total gas pressure (to equalize with outside pressure without increasing partial pressure of reactive oxygen) 3. It can provide a buffer to the nitrate supply.