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I had thought photosynthesis and respiration where pretty much Oxygen and Carbon neutral per calorie.

http://news.mit.edu/2014/technical-feasibility-mars-one-1014

If, as the project plans, crops are cultivated within the settlers’ habitat, Do found that they would produce unsafe levels of oxygen that would exceed fire safety thresholds, requiring continuous introduction of nitrogen to reduce the oxygen level.

Says that a closed system of plants grown to provide calories for human will end up with more oxygen.

Where does the imbalance come from? Where does the extra Carbon go?

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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.

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  • $\begingroup$ I though the free oxygen was accounted for by oil and living things, I guess it also needs to include all the decomposing things. So there is expected to be more oxygen in soil than poop, and even if you burn the stalks the ash is out of circulation for a while, that makes sense. $\endgroup$ – user27830 Mar 8 '18 at 2:35
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    $\begingroup$ Well, in the context of that other question living things = decomposing things... By "decomposition of the plant" we mean that if the carbon in the plant ended up back as CO2 (which happens via decomposition processes, which includes "being eaten and the carbon being part of another organism for awhile"). In practice life is self-perpetuating, so all the carbon doesn't end up in the atmosphere; there is always a certain amount stored in things that haven't decomposed yet, like living organisms and fossilized dead organisms (including oil and coal). $\endgroup$ – Oosaka Mar 8 '18 at 6:14

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