What is the origin of the dominant atmospheric nitrogen content in Earth's atmosphere?
According to the recent paper in Nature Geoscience: Nitrogen speciation in upper mantle fluids and the origin of Earth's nitrogen-rich atmosphere, $N_2$ originates from regions of the Earth where plates are converging.  Venus and Mars lack plate tectonics and therefore lack $N_2$ in their atmospheres.  In other regions of Earth upper mantle, and in Venus and Mars, nitrogen is expected to exist as aqueous ammonium and not enter the atmosphere.