Progress in the ecological impacts of invasive species on soil nitrogen contents and soil enzyme activities following the addition of nitrogen, particularly in different forms, is currently limited. This study aimed to address the ecological impacts of invasive tree Rhus typhina L. on soil nitrogen contents and soil enzyme activities following the addition of four forms of nitrogen, including ammonium, nitrate, urea, and an equal mixture of the three single nitrogen forms. This study also elucidated which form of nitrogen most strongly influenced the competitiveness of R. typhina growth. Aboveground biomass, nitrogen use efficiency, and relative invasiveness of R. typhina under ammonium were greater than those under the control, urea, and mixed nitrogen. Thus, the impacts of ammonium on the competitiveness of R. typhina growth may be optimal compared with other forms of nitrogen. Accordingly, an increase in the relative proportion of ammonium during the addition of nitrogen could improve the competitiveness of R. typhina growth. R. typhina invasion significantly decreased the contents of soil ammonium (under nitrate), nitrate (under nitrate, urea, and mixed nitrogen), alkali-hydrolyzed nitrogen (under urea and mixed nitrogen), and peroxidase activity (under nitrate) in the soil compared with the control. The decreased soil nitrogen contents and soil enzyme activities under R. typhina invasion could be owing to the decrease in soil pH, particularly under ammonium and nitrate. Thus, there is a nitrogen-form-dependence of the ecological impacts of R. typhina invasion on soil nitrogen contents and soil enzyme activities.