Soil microorganisms are key mediators of soil carbon (C) cycling; however several plant and soil related factors potentially influence the functions of soil microbial communities. In that sense, this thesis describes 3 studies which provide new knowledge of factors that influence processes mediated by soil microorganisms. This thesis focused in understanding 1) the mechanisms behind the plant diversity effect on the C transfer from above to belowground; 2) the influence of plant diversity on different soil microbial groups, soil bacterial diversity and phospholipid fatty acid (PLFA) markers evenness; and 3) the relative importance of the combined effects of soil type, seasonal changes and vegetation type. The results demonstrated that plant diversity impacted soil microbial communities directly and indirectly. Root-associated microorganisms were favored directly by plant diversity via increased root biomass, which facilitated access of recently photosynthesized C for this microbial group. In contrast, soil-related microorganisms were favored indirectly by plant diversity. Likely, increased C resource input contributed to the development of soil-related microbial biomass in high plant diversity. Moreover, soil bacterial diversity increased with increasing plant diversity, whilst PLFA evenness decreased. Interestingly, at the highest plant diversity level, PLFAs indicative of root-associated microorganisms predominated; this indicated that plant diversity favors the cycling of recently photosynthesized C. In addition, changing root biomass in different soil types was an important mediator of C resource availability belowground. Whereas, differences in the type of C substrate, from fresh C rhizodeposits to plant litter, related to seasonal changes, were relevant for shaping the microbial community composition. Overall, this thesis contributes to strengthen the knowledge on the role of microbial communities in the C cycling belowground.