Because fossil fuels, such as oil, are naturally formed from dead plant matter, they also contain a smaller ratio of carbon - 13 to carbon - 12. (washingtonpost.com)
It remains unclear whether tropical forests, such as those of the Amazon or Congo, produce the same effect, due to much faster decomposition of dead plant matter in these climes. (scientificamerican.com)
A cow doesn't make methane, bacteria in the cow's gut makes it, and similar soil bacteria and other organisms digest dead plant matter, if not eaten by the cow, to make methane and CO2. (realclimate.org)