Ozone (O₃) pollution can alter carbon allocation and reduce tree growth-both above and below ground, but the effects of direct or indirect interactions among plants, soil and microorganisms under ground-level ozone (O₃) pollution on above- and below-ground carbon sequestration of forests are not yet clear. Here, we used the open-top chamber device and structural equation model to explore the direct and indirect effects of photosynthetic characters (net photosynthetic rate [P_n], stomatal conductance [g_s], intercellular CO₂ concentration [C_i], transpiration rate [T_r], water use efficiency [WUE], excitation energy capture efficiency of PSII reaction center [F_v'/F_m'], actual photochemical efficiency of PSII in the light [PhiPS2], coefficient of photochemical quenching [qP], and apparent electron transport rate [ETR]), leaf and fine root biochemical characters (total carbon, nitrogen, and phosphorus levels, concentrations of non-structural carbohydrates (soluble sugar and starch), lignin, and condensed tannins), soil physicochemical factors (moisture, pH, total carbon, total nitrogen, total phosphorus, and exchangeable NH⁺₄-N and NO^-₃-N levels) and phospholipid fatty acids-based soil microbial community characteristics on above- and below-ground biomass reduction of poplar 107 (Populus euramericana cv. '74/76') at four O₃ concentrations (charcoal-filtered air, non-filtered air, non-filtered air+40 ppb of O₃, and non-filtered air+60 ppb of O₃). The results showed that compared to CF, an increase in O₃ concentration (86 nmol/mol) resulted in a 16%, 11%, and 15% decrease in aboveground, belowground, and total biomass of poplar trees, respectively, with a greater impact on the reduction of aboveground biomass than belowground biomass. Elevated O₃ concentration significantly decreased P_n, g_s, T_r, WUE, F_v'/F_m', PhiPS2, qP, and ETR while increased C_i. The increase of O₃ concentration significantly increased the content of soluble sugar and tannin and decreased the content of starch, TNC and lignin in leaves while had no effects on the contents of C, N, soluble sugar, starch, TNC, lignin and tannin in fine roots. The structural equation model found that photosynthetic rate, leaf starch, and non-structural carbohydrates have a direct impact on aboveground biomass; This indicates that O₃ mainly has a direct effect on aboveground biomass accumulation by affecting the photosynthetic physiological metabolism of plants. The effect of O₃ on belowground biomass is achieved by directly affecting leaf N and total soil carbon, and leaf N in turn affects leaf starch, indirectly affecting underground biomass. Soil pH, total carbon, and soil microorganisms (bacterial and fungal communities) have a direct effect on aboveground biomass, and O₃ has a greater impact on fungal communities than bacterial communities, indicating that plant responses to O₃ through belowground processes can directly feedback on biomass accumulation and allocation processes. This study provides a scientific basis for evaluating the impact of O₃ pollution on the carbon sequestration function of poplar plantations above- and below-ground by analyzing the direct or indirect interactions between plants, soil, and microorganisms.