Subsoil versus Topsoil Straw Addition Shifts Microbial Carbon Allocation for Enhanced Carbon Sequestration and Emission Mitigation

Subsoils (>20 cm) represent an underutilized sink for soil organic carbon (SOC) sequestration because C inputs in subsoil are typically limited. To clarify how organic inputs regulate subsoil SOC accrual and C emissions in rice paddies, we combined a 2-year field experiment with 13C-labeled straw incubation and validated the results with a global meta-analysis. Results showed that compared with no addition and topsoil straw addition, subsoil straw addition increased SOC stocks by 10.5% and 2.2%, respectively, primarily via enhanced mineral-associated organic C (MAOC) formation at 0–30 cm. The 270-day incubation revealed that subsoil addition accelerated microbial conversion of straw C into amino sugars, evidenced by 13C-phospholipid fatty acids, promoting MAOC formation. Relative to topsoil addition, subsoil addition reduced CH4 emissions by 15.3%, driven by improved root growth and enhanced CH4 oxidation, and decreased CO2 emissions by lowering topsoil C availability and peroxidase activity. Meta-analysis confirmed that C emissions decrease progressively with increasing incorporation depth, and machine-learning models indicate that deeper incorporation (>20 cm) mitigates global C emissions by half under future climate change. Overall, compared with topsoil addition, subsoil addition represents a win–win strategy that simultaneously enhances SOC sequestration and reduces C emissions, with potential applicability beyond paddy systems.