Because plants use up oxygen when they decay, or when they’re digested by herbivores, and release CO2 in the process. Plants are carbon neutral for this reason.
Oxygen comes from algae in the ocean. The reason the ocean carbon cycle isn’t carbon neutral is because the carbon is fixed into calcium carbonate by zooplankton that feed on the algae. When these tiny animals die their calcium carbonate based exoskeletons sink and accumulate on the ocean floor, sequestering the carbon there.
Carbon sequestration used to take place on land as well (that’s where all fossil fuels originate) but now we have fungi and animals like beetles that can break down lignin and cellulose instead of allowing it to accumulate.
Oxygen comes from algae in the ocean. The reason the ocean carbon cycle isn’t carbon neutral is because the carbon is fixed into calcium carbonate by zooplankton that feed on the algae. When these tiny animals die their calcium carbonate based exoskeletons sink and accumulate on the ocean floor, sequestering the carbon there.
Carbon sequestration used to take place on land as well (that’s where all fossil fuels originate) but now we have fungi and animals like beetles that can break down lignin and cellulose instead of allowing it to accumulate.