Here's why:

* Abundance: Nitrogen makes up about 78% of the Earth's atmosphere, making it the most abundant gas.

* Cycling: While nitrogen is cycled through various processes, including biological fixation, denitrification, and industrial processes, the atmosphere remains the primary reservoir.

* Other Reservoirs: There are smaller amounts of nitrogen stored in the Earth's crust and oceans, but they are significantly less than the atmospheric reservoir.

Therefore, the atmosphere is considered the main geological reservoir for nitrogen.