Nitrous oxide (N2O) is an important greenhouse gas and participates in the destruction of stratospheric ozone. Soil bacteria produce N2O through denitrification and nitrification, but these processes differ radically in substrate requirements and responses to the environment. Understanding the controls over N2O efflux from soils, and how N2O emissions may change with climate warming and altered precipitation, require quantifying the relative contributions from these groups of soil bacteria to the total N2O flux. Here we used ammonium nitrate (NH4NO3, including substrates for both processes) in which the nitrate has been enriched in the stable isotope of oxygen, 18O, to partition microbial sources of N2O, arguing that a molecule of N2O carrying the 18O labeled will have been produced by denitrification. We compared the influences of six common tree species on the relative contributions of nitrification and denitrification to N2O flux from soils, using soils from the Siberian afforestation experiment. We also altered soil water content, to test whether denitrification becomes a dominant source of N2O when soil water content increases. Tree species altered the proportion of nitrifier and denitrifier-derived N2O. Wetter soils produced more N2O from denitrification, though the magnitude of this effect varied among tree species. This indicates that the roles of denitrification and nitrification vary with tree species, and, that tree species influence soil responses to increased water content.