Fixed Nitrogen

Nitrogen is one of the building blocks plants require to grow.  Nitrogen by itself is too strong of a bond to be usable by plants.  Nitrogen needs to be connected to some other elements to make the outer electron connection easier.  In this case, oxygen.

Places for Fixed Nitrogen

Garden Store

A place to get fixed nitrogen is from fertilizer.  Generally NO3 and NH4 is ammonium nitrate. It's a salt used for commercial fertilizer and explosives.  The salt can be dissolved in water and it splits into Nitric acid and ammonia.  NO3 is directly absorbable by plants, while the ammonia needs to be consumed by  NO4 bacteria and then it is converted into NO3 by another set of bacteria.  When the two are present in a planter, the NO3 gets absorbed by the plants and the ammonia is left on the outside of the pot in the form of white powder.  

Applying ammonium nitrate to plants short-circuits the biological cycle in the soil and eventually renders the soil into a mono-culture.  That's why gardens become more and more dependent on artificial fertilizers.  The soil food web is majorly compromised.

Nature

Another source for ammonium nitrate is nature.  There are several soil food webs that uses the predator/prey cycle with a side effect of releasing fixed nitrogen.  Since adding petroleum based ammonium nitrate to soil really helps plants grow, is it possible to get a similar yield in nature?  The answer is yes.  Below is an example of one predator/prey cycle calculating the amount of ammonium nitrate produced and consumed by plants.