Pinus radiata D. Don is the most widely planted forest species in Chile, making it crucial to understand carbon pools in adult plantations. This study aimed to evaluate the effect of soil type and site productivity on the total carbon stock in adult radiata pine plantations, considering sites with contrasting water and nutrient availability. We selected 10 sites with sandy and recent volcanic ash soils, representing a productivity gradient. At each site, three 1000 m² plots were established to quantify the carbon stock of total biomass using allometric equations and in situ carbon assessments of the forest floor and mineral soil (up to 1 m deep). The results indicated significantly higher carbon stocks in the mineral soil of recent ash sites (281.4 Mg ha⁻¹) compared to sandy soils (139.9 Mg ha⁻¹). The total site carbon was also higher in recent ash (473.2 Mg ha⁻¹) than in sandy sites (330.9 Mg ha⁻¹). A significant relationship was found between stand productivity and soil organic carbon (r² = 0.88), as well as total carbon stock (r² = 0.91) when considering soil type. These findings highlight the importance of including assessments up to 1 m depth and developing soil type and productivity models to improve site carbon stock estimates.