What chemical reaction in the air is capable of forming a
Dense nitrogen compound,or possibly reducing the net pressure of air

The net pressure of air can be lowered by any reaction with oxygen. Oxidation of iron or burning of carbon, for example. All these consume oxygen in the air (approx 20% of air), and thus form a partial vacuum. However, forming a dense nitrogen compound is a bit more complex. I can't honestly think of any reactions that qualify, at the moment.

Most nitrogen reactions involve high pressures and temperatures (formation of ammonia between nitrogen and hydrogen, for example) or they involve electricity in some form (formation of nitrates by lightning or electyrolytic formation of certain chemicals).