The air around us is approximately 80% nitrogen, though it is not available to the plant. Nitrogen is a essential component of the amino acids and proteins in the plant.
Nitrogen does not remain in the plant media in a soluble form for very long and the nitrogen not taken up by plants or micro organisms is easily washed away.

Microorganisms oxidize urea or ammonia into nitrite and then to nitrate. The nitrite form is an intermediate oxidation state that usually quickly converts into the nitrate. The process is not carried out by one form of microbe alone.