How Stable Is the Nitrogen in Earth's Atmosphere

How stable is the nitrogen in Earth's atmosphere?

A) Nitrogen is an inert gas and reacts very little, so it remains essentially constant.
B) Nitrogen is light enough that it is continually escaping Earth's gravity in significant amounts, so it must be replaced by outgassing from volcanoes.
C) Nitrogen reacts with water vapor to form various hydrocarbons. These are subducted under the crust and then outgassed from volcanoes.
D) Bacteria process nitrate minerals and release nitrogen. Atmospheric nitrogen combines with oxygen to eventually form nitrates.