Paddy soils are considered to have a great soil organic carbon (SOC) sequestration potential. The present study was conducted to estimate the amount of new C derived from rice-roots in a paddy soil under global warming with elevated $CO_2$ concentration ([$CO_2$]) using ${delta}^{13}C$ technique. Roots of rice grown with elevated [$CO_2$] were significantly depleted in $^{13}C$ by more than 6% compared to those with ambient [$CO_2$], leading to a low ${delta}^{13}C$ of SOC via rhizodeposition of $^{13}C$-depleted C under elevated [$CO_2$]. The net C storage derived from roots was estimated to be 0.25 and 0.31 kg $m^{-2}$ under ambient and elevated air temperature ($T_{air}$) conditions, respectively. The greater roots-derived C under elevated $T_{air}$ than that under ambient $T_{air}$ collaborated with increased root biomass by elevated $T_{air}$. However, SOC balance analysis revealed that 0.16 and 0.21 kg $m^{-2}$ of autochthonous SOCs were decomposed under ambient and elevated $T_{air}$, respectively, during the growth season. Therefore, elevated $T_{air}$ may enhance incorporation of new C derived from roots to SOC pool due to increased belowground biomass, but warming may also increase decomposition of old SOC by stimulating temperature-sensitive microbial activities.