Biosphere 2 oxygen drop traced to soil microbes and concrete carbonation

A look back at Biosphere 2's 1991 experiment reveals why oxygen inside the sealed Arizona facility fell from 21% to 14% in 16 months. Researchers found that soil microbes consumed oxygen faster than plants could produce it, while concrete walls absorbed much of the resulting carbon dioxide through a process called carbonation. This closed-system study shows how building materials can act as carbon sinks, though the uncertainty in the measurements was significant. For anyone interested in carbon accounting or low-carbon construction, the concrete carbonation finding is the most relevant takeaway. It highlights a natural CO2 uptake mechanism that is often left out of lifecycle assessments. However, the authors noted a wide margin of error in their estimates, and the effect may be slower in typical buildings with lower indoor CO2 concentrations. The story also underscores how tight sealing and careful engineering are needed to observe slow atmospheric changes accurately.