Oxygen balence from growing plants for food
If oxygen and respiration were carbon and oxygen neutral, there wouldn't be free oxygen at all. Plants would just respire all the oxygen they produce.

The extra carbon goes into the plants themselves. It isn't just what they use for energy, it is also what they are made of.

See also this question:

Are plants really oxygen neutral?

Plants are oxygen neutral when we take into account the decomposition of the plant, i.e. take the long-term view where the carbon making up the plant gets back to being CO2.