All life is made of carbon. Plastic is made from hydrocarbons sourced from the remains of ancient microbial life. Generally there are fewer emissions associated with e.g. a tree decomposing than that same tree being burnt, since the organic (meaning carbon-based in this case) molecules are either used by the decomposers, or broken down into their constituent parts, eventually becoming soil. I'm not sure if there is bacterial respiration involved here that would mean significant emissions.