Question

Where is most nitrogen found?

Answer 1

atmosphere

Explanation:

Nitrogen is notoriously difficult to remove from the atmosphere (~80% of air is inert nitrogen gas). Only nitrogen fixing bacteria, lightning and modern industry (via Haber-Bosche Cycle) remove nitrogen from the air so that it can be made available to living systems (protein and DNA are dependent on nitrogen).

nitrogen returns to the atmosphere via denitrifying bacteria, making it (the atmosphere) the largest reservoir. Living things would represent the next largest reservoir.