Aims <p>Soil organic carbon (SOC) accumulation and decomposition processes are influenced by litter quality and inputs. Understanding how tree species composition influences soil C dynamics is essential for improving forest carbon sequestration since species mixing with different functional characteristics can enhance SOC stocks through complementary mechanisms. We evaluated SOC stocks, decomposition, and respiration rates in mixed and monospecific stands of Scots pine and Pyrenean oak in mountains of the Iberian Peninsula.</p> Methods <p>We sampled forest floor (Ol and Oh + Of) and mineral soil (MS-A) layers in mixed and monospecific stands and evaluated SOC stocks. Nitrogen content and glucosidase activity were also analyzed in MS-A. Soil respiration and decomposition rates were assessed using the Soda Lime and Tea Bag Index methods, respectively.</p> Results <p>C stocks were greater in Ol layer in mixed than in monospecific stands. However, C stocks in the Oh + Of layer was greater in pine and mixed stands than in oak ones. Monospecific pine stands showed more limiting conditions for organic matter decomposition and biological activity of soil organisms as indicated by lower respiration rates and higher soil C/N ratio and stabilization factor of the Tea Bag Index method.</p> Conclusions <p>Species identity played a more prominent role in soil functioning than species mixing, with pines and oaks showing contrasting effects on C stocks and soil respiration. Our results also suggest that oak admixture into monospecific pine stands may mitigate limiting conditions for organic matter decomposition while maintaining high C stocks and enhancing soil multifunctionality.</p>