Good comments dead_sled. I see you are thinking!

Quote Originally Posted by dead_sled
With AP, we are not a direct part of the system.
False. Explain why you think this must always be true? You state this as a priori knowlege, yet I would question the basis for your assumption.

Quote Originally Posted by dead_sled
Our waste products do not stay in the system.
Some waste products certainly do stay in the system, but some are removed; therefore, this statement is partially false.

Quote Originally Posted by dead_sled
How is the loss balanced without human input?
It depends. The loss of what? Carbon, hydrogen, and oxygen are the bulk of plant matter. These can come from "the air" for most AP systems, if so designed. In fact, some are. As for the rest, it depends because it's very specific to the details of the system.