Light and soil nitrogen availability can be strong controls of plant nitrogen (N) fixation, but data on how understory N-fixing plants respond to these drivers are limited despite their important role in ecosystem N cycling. Furthermore, ecosystem N cycling can be altered by the introduction of species with nutrient use patterns that differ from natives. We assessed how N fixation of two exotic, understory species responded to varying light and soil N environments. We sampled leaf tissue from Mimosa pudica L., Desmodium triflorum (L.) DC., and a non-fixing reference plant (Axonopus) growing in control and two N fertilization treatments under either N-fixing or non-N-fixing trees, which may alter local soil nutrient cycling, across a range of light conditions. We measured N fixation with 15N isotope dilution, and ensured that N-fixing neighbor trees were in fact fixing N. All understory plants were wild-growing species not native to the study location. Desmodium and Mimosa acquired 82.6% and 71.6% of their nitrogen from fixation (%Ndfa) in the control, compared to 66.8% and 58.1% in the +10 g N m−2 y−1 treatment and 73.1% and 64.7% in the +15 g N m−2 y−1 treatment. These subtle %Ndfa differences across fertilization treatments were more apparent at low light availability and disappeared at high light availability. The amount of N fixed by neighboring trees did not influence %Ndfa in the understory species. Our study shows some differences in N fixation across different nutrient environments at low light for two N-fixing species, though the changes were small, and both species derived most of their N from fixation. These findings imply that introduced N-fixing species could exacerbate ecosystem N enrichment, particularly under high soil N conditions