For earthworms it is species specific (both  regarding earthworms and litter types). But see more in this paper.

                    Csaba

Thanks for the paper. It is full of interesting informations. It appears that it is commonly known that earthworms use litter predigested by fungi and/or bacteria, except for the Hornbeam thet is directly drown into the burrow. Do I have well understood? Why Hornbeam litter is digested easier by earthworms?

Yes, sure. Hornbeam, because of the lower tannin content is  rightly palatable for earthworms. In harder leafs like beach or oak the tannin content is higher which prevent the direct consumption. They became palatable after some microbial decomposition.

Thanks. I will ask a new question in another post about earthworm and sand.

This point seems to be inderstood thanks to you! I have understood that the first step of biodegradation is taken in charge by bacteria and/or fungi. Then eartworm ensure the bioturbation work, eating OM with a C/N of 25 to 50 depending on their species. The C/N of their dejection is about 20 [Bouche] which is more than the humus C/N. So the last step is done by bacteria or/and fungi, getting the OM passing from a C/N of 20 to 10.

We have noticed the cold compost stop the processus at a C/N of about 20, whereas a warm compost stop at 10. This is the next question too. 

Earthworms can consume most organic materials that initial C/N ratio around 30