Ground-level ozone (O) pollution often co-occurs with anthropogenic nitrogen (N) deposition. Many studies have explored how O and soil N affect aboveground structure and function of trees, but it remains unclear how belowground processes change over a spectrum of N addition and O concentrations levels. Here, we explored the interactive impact of O (five levels) and soil N (four levels) on fine and coarse root biomass and biomass allocation pattern in poplar clone 107 (Populus euramericana cv. '74/76'). We then evaluated the modifying effects of N on the responses of tree root biomass to O via a synthesis of published literature. Elevated O inhibited while N addition stimulated root biomass, with more pronounced effects on fine roots than on coarse root. The root:shoot (R:S) ratio was markedly decreased by N addition but remained unaffected by O. No interactive effects between O and N were observed on root biomass and R:S ratio. The slope of log-log linear relationship between shoot and root biomass (i.e. scaling exponent) was increased by N, but not significantly affected by O. The analysis of published literature further revealed that the O-induced reduction in tree root biomass was not modified by soil N. The results suggest that higher N addition levels enhance faster allocation of shoot biomass while shoot biomass scales isometrically with root biomass across multiple O levels. N addition does not markedly alter the sensitivity of root biomass of trees to O. These findings highlight that the biomass allocation exhibits a differential response to environmentally realistic levels of O and N, and provide an important perspective for understanding and predicting net primary productivity and carbon dynamics in O-polluted and N-enriched environments.