As I understand it your question is how much plants are needed to absorb the
CO2 breathed into the air by a single human?

I suppose you could calculate a minimum by starting with the number of
molecules of CO2 breathed out each day, and do a calculation of how much
plant this amount of C would translate to. The total amount of plant needs
to increase by at least this much each day. If this is continued ad
infinitum the amount of plant will increase ad infinitum also.

No matter what you do it will be a highly hypothetical result. In every
imaginable case other factors will be more important in determining the
hypothetical design.
PvR

schreef
How much plant biomass is needed per human?

In the first place, someone will (correctly) respond that the question
is poorly conceived, hopelessly vague, dependent on too many unknowns,
etc.

Granted. Cleaning up the question will probably be the first step
towards finding a meaningful answer.

So let's start with these:
* I'm referring to O2 and CO2 exchange only; not using plants as
food, fuel, building materials, etc.
* We can all recognize that humans have varying needs. One size does
not fit all.
* The O2 output per gram of one plant is probably wildly different
than that of another species.

Asked another way:
Assume that one normal healthy adult human is placed in a sealed
environment with everything needed, except a renewable oxygen supply,
and he wants to live there indefinitely. How much plant life should
be included in this environment? (Ignore the fact that the artificial
environment will almost certainly degrade quickly due to other
causes.)

Thank you!

Ted Shoemaker

P.S. No, this isn't a homework assignment. I'm just trying to learn.