Plastics are made from organic molecules. They are quite susceptible to heat and light. Since the damage goes all the way down to the level of molecular bonding, I would surmise that the smaller decomposition products are susceptible to bacterial metabolism.
I wonder what the soil analysis on samples taken near areas where plastics are decaying would reveal.
Many years ago General Motors did a research on where the rubber from tires went as tires wore down. They could not find a trace of rubber on the side of the road.
One would think that with all the millions of tires rolling down the road there should huge mounds of rubber dust piled up but there was none.