And in 4 billion years, that layer of microplastics will have condensed down into fuel for the next creatures to annihilate their atmosphere with. Circle of life.

Did those trees, before there were decomposers, have access to nitrogen fixing bacteria? Where were they getting ammonium and nitrate?
Just stuff built up from lightning, nitrogen and oxygen?

Edit: Looks like land dwelling soil forming bacteria started in the Cambrian. Then, in the Ordovician the first land plants. Then, in the Silurian vascular plants and trees appeared.