Application of sewage sludge in sylvo-pastoral ecosystem is a novel way of valorisation. The effects of this addition on microorganisms have been studied in a cork oak litter (Quercus suber L.). A field experimental design was carried out during fourtheen months using litter-bag method both on control and treated plots (Var, France). The results showed that the sewage sludge addition stimulated the microbial activity immediatly and modified the genetic structure of bacterial community. The stimulation has been shown by a significant increase of real and potential respirations, bacterial and fungal numbers, and enzyme activities (i.e. laccases, peroxidases and cellulases). However, the resilience of microorganisms and of their activities occured quickly consequently to sludge addition. Moreover, sewage sludge had no effect on the seasonal dynamics of microbial activity during the decomposition of litter. Indeed, the litter humidity remained the prevailing factor in the annual evolution of indigenous microbial communities. Thus, sewage sludge addition modifies the intensity of microbial responses to environmental factors (e.g. moisture) with a conservation of biological patterns regulating the decomposition of litters.