What chemical reaction is capable of converting atmospheric nitrogen spontaneously into a dense nitrogen compound,not necessarily useful to biology.

Production of ammonia (it's spontaneous -- at high temperature and pressure).

N_2 + 3H_2 \rightarrow 2NH_3

Nitrogen won't react with many things. It's often used as an "inert gas", though it isn't anywhere as inert as, for example, argon.