Decomposition is the process of degradation or break down of organic matter such as cellulose or other polysaccharide to simpler unit or monosaccharide. Usually decomposition takes place in absence of oxygen such as anaerobic decomposition. The decomposition results in evolution of carbon dioxide by degrading the C , H and O of organic matter with the help of bacterias ro microorganisms.
Thus, with decomposition, the level of carbon dioxide increases.