Final answer:
Oxygen returns to the atmosphere from the biosphere through photosynthesis, a process where plants and certain other organisms use sunlight to produce oxygen and organic compounds, significantly contributing to the biogeochemical carbon cycle.
Step-by-step explanation:
Oxygen Reentry into the Atmosphere via the Biosphere
Oxygen reenters the atmosphere from the biosphere primarily through the process of photosynthesis. This crucial process involves photosynthesizing organisms, such as land plants and marine phytoplankton, which use sunlight to convert carbon dioxide and water into glucose and oxygen. While respiration, transpiration, and decomposition are important biological processes, they consume oxygen and release carbon dioxide, rather than releasing oxygen into the atmosphere. Through photosynthesis, not only is oxygen produced and released, but carbon is also sequestered in the form of organic compounds, playing a vital role in the biogeochemical carbon cycle. This cycle includes the biosphere, where the interchange of carbon dioxide and oxygen sustains life on Earth, providing oxygen for organisms to undertake aerobic respiration.