Do grocery bags decompose faster in the ground?

Plastic bags, despite critical claims by ecological groups, do decompose.

Originally the polyethylene in plastic grocery and garbage bags took a very long time to decompose (as other plastic items still do). However, the addition of various substances to the bags now causes the material to fragment, and it is then slowly broken down by natural processes. Early additives required sunlight (UV), but heat and moisture are now key to the degrading process.