1) Break down 1) Rot 
To breakdown organic substances, using microorganisms.
biodegrade 1. To decay and to become absorbed by the environment. 2. The conversion of molecules from one form to another within an organism, often associated with change in pharmacologic activity; refers especially to drugs and other xenobiotics. 3. The series of processes by which living systems render chemicals less noxious to the environment....
verb break down naturally through the action of biological agents; `Plastic bottles do not biodegrade`
to decay and become absorbed by the environment: toys that will biodegrade when they're discarded.