The complex organic molecules are broken down by them to get energy.
The individual components of these compex molecules are SET FREE in the environment ( air, soil , water ).
Thus these are now FREE for recycling .
example - A protein molecule is a very compex and big one with C,H,O, AND N in addition it may contain other atoms such a Fe , S as well ! It has some energy stored in it .
When it is decomposed , all the individual elements mentioned above in capital letters are released for recycling .
This is equally true for a dead tree .