There were some changes in the dominant species, but mountaintop microbes decomposed dead grass just as well as desert microbiomes in the hot, dry climate.
He took a severed head from the lunatic asylum and kept it on the roof of his studio for two weeks, so that he could draw its features as it decomposed.
That log is being decomposed by bacteria, among other things, that take in oxygen, and release CO2. But in the Carboniferous, those wood-eating bacteria didn't exist yet.