You eat plant life and, in a natural system, your excretia becomes the mulch that feeds the plants you eat. Trash is a product Life knows how to use. Life grabs opportunities. Excess trash of any kind will make an opening for any new form of life that can figure out how to use it. Expect, within the next few thousand years or so, the arrival of the Huggies Monster and the Tire Mound Scavenger.
Life likes volatiles.
Stable elements like nitrogen are never going to be the basis of a life form. You need unstable stuff, that Life can mix and match like tinkertoys. Any nice moderately unstable stuff will do, however. The smaller and weaker and tastier you are, the more kids you must have all at once to guarantee survival of the species. If you are a member of a species that gives birth to snacks at the point where they are most likely to be devoured, you'll have millions of children at a time.