Question
Why do you need Nitrogen for photosynthesis?

Answer
Take a good look at this Chlorophyll "a" Molecule:

http://scifun.chem.wisc.edu/chemweek/chlrphyl/chlrphyl.html

See any Nitrogen?

There are different kinds of Chlorophyll.  The one in the illustration above is the most common molecule, Chlorophyll a, found in three-fourths of the world's green plants.  But they have one basic thing in common: All of them take rays from the sun and turn it into energy, turning CO2 and H2O (Carbon Dioxide and Water) into Carbohydrates (Glucose/Sugar) and Oxygen.

The chemical reaction for Photosynthesis is: 6CO2 + 6H2O (+light energy) -- --> C6H12O6 + 6O2.

But without Chlorophyll, you cannot accomplish this.  Without Nitrogen, you cannot make Chlorophyll.

It may seem like we don't need Nitrogen - after all, we need Oxygen to breathe, not Nitrogen.  But 78 percent of the World is Nitrogen.  This is an essential element for the most basic biological processes.  It is in Amino Acids; it makes up Proteins and Nucleic Acids, among many other things.   Nitrogen cannot be "fixed" without Oxygen to generate energy for N2 fixation. Too much Oxygen, however, inactivates the enzyme nitrogenase.

So you see, Nitrogen is central to life not only for Plants, but for Animals as well.