Biogeochemical cycle

Question 1

What is the biggest nitrogen reservoir

Answer 1

the atmosphere, at: 3.5×1018kgNy−1

Question 2

If there is such an abundance of nitrogen in the atmosphere, why can it be a limiting factor for nitrogen?

Answer 2

Nitrogen exists in the atmosphere as a triple bond. It must be broken down by nitrogen fixing bacteria, in order for vegetation to use it.