The whole system is hermetically sealed.  There is a place off to the side on the property where the air inside the system has place to expand, it's called the lung.  Pictured below is the metal weight in  the center of a circular tent keeping pressure up.  The white bars are supports when the whole tent rests on the ground.  The whole ceiling goes up and down depending on the air pressure.

 

This is a picture of the tunnel/passage of air going to and from the lung.  It can be very windy.

Chambers in the AZ Biosphere

Sections of the green house was sectioned off as dessert, ocean, rain forest and etc...  Everything is planted in some sort of cement flowerbed.  I'm not clear how deep they go.

The whole greenhouse acts like an optical lens.  The sun light is captured and heats up the greenhouse. There is a lot of effort put into the basement of AZ Biosphere II for chilling the air.  Pictured below is a section of their A/C.  The water tanks and pipes for chilling and recovering water from plant beds is on the right.  The air heat exchangers are on the left.  

 

 

Biological Cycles

While visiting the place, it did not appear much interest in growing soil or understanding how chemical cycles worked in 1993 - 1994.  Since the University took it over , there appears to be more research in that area.  Right now the University is using it as a classroom and general research on plants, soil and the like.

Results

From everything I read and saw, I'm not really clear if they could live inside for an extended time without extra O2.  It appears the project was sabotaged by people not getting along, rather than Mother Nature preventing them from growing food in a confined area.

Hindsight is always 20/20.  They should have started small, and perhaps with a rat or so. ;-)  The amount of information they received for the cost is way too low.

If you have a chance to visit the place, it's a good thing to do.  It takes about 2 hours.  And there are tours.