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We undertook a study to determine if non-native earthworm populations occurring in sugar maple (Acer saccharum Marsh.) forests of eastern Canada could potentially alter denitrification rates driven by bacteria and fungi. We measured earthworm abundances and collected surface mineral soil samples from 38 sugar maple forests, 14 that were colonized by earthworms (EW+) and 24 that were earthworm-free (EW-). In each soil sample, we measured (1) fungal, bacterial and total microbial biomass, (2) fungal, bacterial and total potential denitrification, (3) the abundances of bacterial and fungal denitrifying genes, and (4) soil physicochemical properties known to influence denitrification rates. Earthworms decreased forest floor depth and soil C:N ratio, but increased mineral soil pH, total N, bacterial and fungal biomass, and nitrification rates. Earthworms increased bacterial-driven denitrification more so than fungal-driven denitrification. Accordingly, specific denitrification rates (i.e., denitrification-to-biomass ratio) increased for bacteria and decreased for fungi with the presence of earthworms. The relative abundances of bacterial denitrifying genes (nir K, nir S and nos Z) increased with the presence of earthworms, whereas the fungal denitrifying gene (P450 nor) was not affected. Taken collectively, our results suggest that earthworms increase N 2 O emissions in sugar maple forest soils mainly by promoting the bacterial denitrification pathway. This increase is due to physicochemical changes in the soil environment promoting bacterial activity and to changes in the functional diversity of bacterial communities. • Sugar maple (Acer saccharum) forests occur with (EW+) or without (EW-) earthworms. • Nitrification, pH, fungal and bacterial biomass were higher on EW + sites. • Earthworms increased bacterial N 2 O emissions more than fungal N 2 O emissions. • Bacterial denitrifying genes were more abundant on EW + sites. [ABSTRACT FROM AUTHOR]