Once combustion begins, the high levels of oxygen drop as free oxygen is consumed. This counterracts conditions favorable for fire, and fire therefore acts as a negative feedback mechanism to rising levels of oxygen gas. If oxygen levels rise, fire is more likely to occur which then starves itself by driving oxygen levels to lower concentrations. When O 2 levels rise, even as little as a few percent, the conditions for combustion are greatly enhanced. Fire, therefore, places an upper limit on the amount of atmospheric oxygen that is stable by serving as a negative feedback mechanism which counterracts any appreciable rise.
Fire depletes oxygen directly, but also promotes the activities of other oxidation processes.
By exposing fresh ground to weathering and erosion, the demand upon atmospheric oxygen is increased. Fire strips the ground of vegetation, allowing rains to erode fresh rock and soils. Reduced matter that had been locked up in the Earth is then adding to the demand upon atmospheric oxygen through the reactions of chemical weathering.