Estuarine air-sea CO₂ flux is governed by a mix of terrestrial inputs that have opposing effects, yet the quantitative control of these inputs remains unclear, complicating predictions under anthropogenic perturbations. To address this, we used an observationally constrained physical-biogeochemical model to conduct a process-based attribution of air-sea CO₂flux in the eutrophic Pearl River Estuary during summer, when peak river inputs drive the most active carbon cycling of the year. Results show that the system acts as a net source because CO₂-enhancing processes (the input of high-pCO₂river water and terrestrial organic matter (OM_terr) remineralization) dominate over nutrient-fueled CO₂ consumption. The estuary-wide air-sea CO₂ flux is an order of magnitude more sensitive to changes in OM_terr load than to nutrient variations. Crucially, OM_terr stoichiometry is a key regulator: a lower C:N ratio enhances a compensation effect, where remineralization releases nutrients that stimulate CO₂ uptake, partially offsetting emissions. Specifically, across the C:N range of 6 to 12, every 2-unit decrease reduces CO₂ outgassing by ~14%. This suggests that the observed widespread decrease in the C:N ratio of OM_terr has likely attenuated estuarine CO₂ emissions. Therefore, managing both the quantity and stoichiometric quality of terrestrial organic matter inputs, alongside riverine carbonate chemistry, is paramount for mitigating estuarine CO₂ emissions.