The Biosphere 2 experiment in the 1990s attempted to create a closed ecological system in the Arizona desert. The project faced a critical failure when oxygen levels dropped unexpectedly, threatening the health of the eight residents. Scientists discovered that soil microorganisms consumed oxygen faster than plants could replace it. Additionally, the concrete used in the structure acted as a carbon sink, absorbing carbon dioxide and disrupting the expected atmospheric balance.
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The part about concrete absorbing CO2 is actually a known chemical process called carbonation. It is interesting to see it cause a crisis in a closed loop, but in the real world, this is why some people argue that the built environment acts as a permanent carbon sink.
True, but the scale matters. In a massive building, you get some sequestration, but the emissions from producing the cement in the first place usually dwarf the amount it absorbs over its lifetime. The net balance is almost always negative.
This is a classic example of failing to account for all system inputs and outputs. They modeled the plants and humans but ignored the chemistry of the building materials and the soil biology. It is the same mistake people make with some current energy storage projections.
