What "sphere" is the largest reservoir of nitrogen?

The atmosphere

Atmosphere is a thin layer of gases that floats over the surface of the planet. The atmosphere is composed of 78% nitrogen, 21% oxygen, and 0.93% argon. The remaining 0.07% consists of water vapor, carbon dioxide, ozone and noble gases. The reason for the abundance of nitrogen in the atmosphere is because it does not react or bond with other elements.