<p>Rice dead roots are the primary sources of soil organic matter (SOM) in paddy fields. However, the decomposition of dead roots in rice-growing soils and its effects on native SOM priming under elevated CO₂ concentration (eCO₂) and warming remain unclear. In this study, we investigated the effects of increased living root biomass under eCO₂ and warming on the decomposition of dead roots, along with concurrent changes in CO₂ from the SOM decomposition and living roots. ¹³C-labeled dead roots were prepared, and the rice growing experiments were conducted with and without dead roots. Dead root decomposition was determined by analyzing δ¹³C of CO₂ evolved during the rice growing period. The decomposition rates ranged from 58.4 to 88.0%, with enhanced decomposition noted under eCO₂ with warming. The decomposition rates were Linearly correlated with Living root biomass, which also increased by 23–131% under eCO₂ and/or warming. Furthermore, the dissolved organic C concentration and microbial biomass C also linearly increased with living root biomass. These results suggest that increased living root biomass under eCO₂ and warming increased rhizodeposits, stimulating microbial activity to decompose labile dead roots. Soils with dead roots exhibited lower CO₂ emissions from native SOM and living roots than those without dead roots. This indicates the negative priming of recalcitrant SOM, likely due to the preferential decomposition of labile dead roots by microbes. Our findings suggest that eCO₂ with warming increases the production of root residues; however, they also enhance the decomposition of dead roots while decreasing SOM priming.</p>