what would happen is there was no nitrogen in the atmosphere.

Nitrogen makes up more than three quarters of the air we breath. So without it, there would be substantially less air, so density of air would decrease. Nitrogen is present in DNA, urine, gases, almost everything living has nitrogen in it. If there is no nitrogen in atmosphere, so air would now mostly comprise of oxygen and carbon dioxide, lots of animals and living creatures would die because of the incredibly high concentrations of these gases. Nitrogen is also used by plants for photosynthesis. If only oxygen would have been present on the earth, then only one man can explode the whole earth using a single lighter. So, nitrogen plays very important part in balancing the earth's atmosphere.