Interactive effects of ozone exposure and nitrogen addition on tree root traits and biomass allocation pattern: An experimental case study and a literature meta-analysis.

Ground-level ozone (O 3) pollution often co-occurs with anthropogenic nitrogen (N) deposition. Many studies have explored how O 3 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 3 concentrations levels. Here, we explored the interactive impact of O 3 (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 3 via a synthesis of published literature. Elevated O 3 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 3. No interactive effects between O 3 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 3. The analysis of published literature further revealed that the O 3 -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 3 levels. N addition does not markedly alter the sensitivity of root biomass of trees to O 3. These findings highlight that the biomass allocation exhibits a differential response to environmentally realistic levels of O 3 and N, and provide an important perspective for understanding and predicting net primary productivity and carbon dynamics in O 3 -polluted and N-enriched environments. Unlabelled Image • Root biomass and biomass allocation were studied for poplar at 5 O 3 and 4 N levels. • Elevated O 3 inhibited but N addition stimulated root biomass. • R:S ratio was decreased by N addition but not affected by elevated O 3. • No interactive effects of O 3 and N on root and R:S ratio. • Soil N addition did not ameliorate O 3 -induced reductions in root biomass. [ABSTRACT FROM AUTHOR]