C6H12O6 + 6O2 → 6CO2 + 6H2O . This means carbon is produced.
Photosynthesis is what plants do to create glucose the equation is:
6 CO2 + 12 H2O → C6H12O6 + 6 O2 + 6 H2O. This means carbon is used.
Decomposition is happens when an animal dies, it is then eaten by a decomposer which releases the carbon in it back into the atmosphere.
Combustion is burning, if something with carbon is burnt it will release it into the atmosphere, e.g. a tree, fossil fuel.
This is one of many useful diagrams: