What is necessary for nitrogen in the air to combine with oxygen?
Hmmm, are you asking why we treat air as an homogeneous mixture?
Of course, in a gaseous mixture, the gaseous molecules are more or less independent of each other; they occupy a LARGE volume and are LOW DENSITY materials in comparison with the condensed phases.
In air, we can speak of the partial pressure of each component.
And using the Ideal Gas equation:
Anyway, see here for similar treatment.