Temperate forest soils are net sources of carbon dioxide (CO 2 ) and net sinks for methane (CH 4 ), the two greenhouse gases most responsible for contemporary global climate change. Both soil carbon fluxes are sensitive to their local tree communities due to the direct effects of tree traits as well as indirect effects of associated soil properties. We asked how tree species identity and diversity predicts the flux of CO 2 and CH 4 from soils, how the two net fluxes are related, and what tree and soil characteristics predict their magnitudes. In a mixed temperate forest in central Massachusetts, we established 49 plots containing either a single tree species or a combination of those species and measured growing season soil CO 2 and CH 4 fluxes for two years. We found generally greater soil CO 2 and CH 4 fluxes associated with deciduous tree species. CH 4 uptake rates were more sensitive to tree species than were CO 2 fluxes. Tree species mixtures lead to predictable intermediate fluxes of CO 2 , but mixtures resulted in lower than predicted CH 4 uptake. Soil CO 2 emission and CH 4 uptake were both positively related to total litter inputs. Soil CO 2 emission was additionally associated with warmer temperatures and a lower ratio of soil carbon to nitrogen; in contrast, CH 4 uptake was associated with lower soil moisture and a shallower organic horizon. Thus, tree species community composition may prove useful for predicting soil carbon fluxes, but much remains to be discovered about the mechanisms linking tree species to associated microbial and biogeochemical processes.