The stimulus to organic matter cycling and, consequently, the increase in carbon (C) and nitrogen (N) pools in the soil are the most important benefits in mixed Eucalyptus plantations with N-fixing trees. However, how bacteria community can contribute to a better C and N dynamics remain poorly understood in this type of forest management. Here, we evaluated interactions between bacteria community and C-N functions in soil and litter interface resulting from pure and mixed Eucalyptus grandis and Acacia mangium plantations. Overall, we hypothesized that bacteria community (diversity, structure, composition and genes abundance) is different and closely related to C-N improvement in that mixed plantations. We sequenced the 16S rRNA gene (V4 region) using 515F and 806R primers into MiSeq Illumina® sequencing platform and assessed the link between bacteria community and C-N functions (i.e, microbial and enzymes activity, functional genes and soil-litter nutrient cycling). Soil and litter samples from E. grandis without (E) and with N addition (E+N), a mixed system between E. grandis and A. mangium (E+A) and a pure A. mangium (A) treatments were collected at 27 and 39 months after planting. We found that mixed plantation (E+A) increase bacterial diversity (soil and litter) and gene nifH (soil) abundance, while pure E. grandis reduced it (especially E+N). Also, we find a strong correlation between Rhizobium and Sphingomonas with nifH and soil N contents (especially at 27 months). The Total-N content may have regulated bacterial structure, mainly at litter interface. Our study provides a novel evidence of the importance of A. mangium in a mixed system with E. grandis for C-N cycle in sandy soils conditions, with low organic matter content. It is extremely important because can minimize the mineral fertilizers use and improve soil and plant health.