Why is the amount of nitrogen that cycles among organisms much smaller than the amount in the atmosphere?

Question

Quizplus · Accepted Answer

Nitrogen gas in the atmosphere cannot be used directly by most organisms. It needs to be converted into other forms such as ammonia or nitrate through nitrogen fixation before it can be utilized by organisms. This conversion process limits the amount of nitrogen that can cycle among organisms.

Why Is the Amount of Nitrogen That Cycles Among Organisms