The addition of broad-leaf trees to conifers has many purposes including reduction of soil acidification, increase in litter turnover and increase in fertility. The purpose of this study was to determine the impact of mixing broad-leaf trees with coniferous trees on the bacterial and fungal communities and functional genes that govern litter decomposition. In larch, sassafras and mixed larch/sassafras plantations, litter samples were collected 60, 150, 270, and 360 days after peak leaf fall. The abundance of the bacterial classes Acidobacteriia and Solibacteres, and their functional genes for carbohydrate and amino acid metabolism, were less abundant in the mixed plantation than in larch plantation. In contrast, the fungal class Dothideomycetes and its functional genes for carbohydrate and amino acid metabolism were more abundant in the mixed plantation compared to the larch plantation. Litter pH and total carbon concentration were major determinants of bacterial and fungal community structure and the abundance of functional genes. Overall, we provide evidence that co-culture of sassafras with larch increased the abundance of potential functional genes of Dothideomycetes responsible for degrading organic matter but reduced those of Acidobacteriia and Solibacteres.