How does poop impact the nitrogen cycle?

When animals digest protein, some of the nitrogen from that protein ends up in our poop and pee. Bacteria break the waste down, turning that nitrogen back into ammonia. ... In the water, ammonia acts like a fertilizer for tiny microscopic organisms called phytoplankton.

Well, animals digest protein. And some of the nitrogen from that protein ends up in the poop and pee. They are then broken down by decomposers and the nitrogen is re-introduced into the soil in the form of ammonia. 
-hope it helps :3