There are three categories of biogeochemical cycles. These are hydrologic cycles, atmospheric cycles, and sedimentary cycles. I am assuming you all have learned about the hydrologic cycle and the carbon cycle.
Nitrogen enters ecosystems primarily through the actions of nitrogen fixing bacteria.
Some nitrogen fixing bacteria live independently, but most live in association with the roots of certain plants. Animals obtain nitrogen directly or indirectly from plants. The nitrogen used by animals is ultimately either recycled for use by other organisms or returned to the atmosphere. Ammonifying, denitrifying, and nitrifying bacteria are essential for these processes. Humans in the developed world have switched, over the course of the past century, from a reliance on biological fixation of nitrogen to reliance on industrial fixation of nitrogen.