Ground-level ozone (O3) pollution often co-occurs with anthropogenic nitrogen (N) deposition. Many studies have explored how O3 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 O3 concentrations levels. Here, we explored the interactive impact of O3 (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 O3 via a synthesis of published literature. Elevated O3 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 O3. No interactive effects between O3 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 O3. The analysis of published literature further revealed that the O3-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 O3 levels. N addition does not markedly alter the sensitivity of root biomass of trees to O3. These findings highlight that the biomass allocation exhibits a differential response to environmentally realistic levels of O3 and N, and provide an important perspective for understanding and predicting net primary productivity and carbon dynamics in O3-polluted and N-enriched environments.
Keywords: Isometric allocation; Nitrogen addition; Optimal partitioning; Ozone; Root biomass; Root:shoot ratio.
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