Can't get much more biodegradable than what we do here in israel: a bedsheet shroud in a meter and a half deep hole in the ground.  No coffin, but cement flagstones are used to outline the bottom of the hole, and after the body is placed inside, other cement flags are used to pave over the top.  Then the dirt is shoveled on top of this.
Part of this is because Jewish ritual law requires that the body be buried on the same day that it died.
Djughashvili's Gerbil

Ironrodent aka Djughashvili's Gerbil