Probably exactly the right amount of Nitrogen in the environment.
The oxygen content rose to around 35% before animals evolved and spread, So nitrogen was down to 65%. Result was that everything on Earth was about twice as combustible as now, so huge fire-storms raged across the planet periodically and burnt off most of the vegetation.
Having 78% of the atmosphere as a benign, non-poisonous, non-reactive filler is an excellent design, and I commend it to whoever has responsibility for designing the planets nowadays.
Where it came from (in the atomic sense), I don't know. Hydrogen (Atomic Weight 1) and some Helium (AW 4) came from the Big Bang. Helium is formed in stars in the fusion process. Carbon (AW 12) and Oxygen (AW 16) form reasonably well from Helium in various stages of the life of a star. Fred Hoyle did a lot of work in accounting for the proportions in which various elements were formed. Nitrogen (AW 14) probably should not be very plentiful, but there may be some favourable state of the atomic nucleus to account for it.
The other question is, what solid compounds have it as a component, and is the atmosphere in balance with anything? Nitrogen forms a cycle through nitrogen-fixing plants, and a lot of it ends up in bird guano, but that hardly accounts for a large-scale equilibrium. It just seems to sit there diluting all that oxygen.
