We compared soil organic carbon (SOC) stocks and stability under two widely distributed tree species in the Mediterranean region Scots pine (Pinus sylvestris L.) and Pyrenean oak (Quercus pyrenaica Willd.) at their ecotone. We hypothesised that soils under Scots pine store more SOC and that tree species composition controls the amount and biochemical composition of organic matter inputs, but does not influence physico-chemical stabilization of SOC. At three locations in Central Spain, we assessed SOC stocks in the forest floor and down to 50cm in the mineral in pure and mixed stands of Pyrenean oak and Scots pine, as well as litterfall inputs over approximately 3 years at two sites. The relative SOC stability in the topsoil (0-10cm) was determined through size-fractionation (53μm) into mineral-associated and particulate organic matter and through KMnO4-reactive C and soil CN ratio.Scots pine soils stored 95-140Mgha-1 of C (forest floor plus 50cm mineral soil), roughly the double than Pyrenean oak soils (40-80Mgha-1 of C), with stocks closely correlated to litterfall rates. Differences were most pronounced in the forest floor and uppermost 10cm of the mineral soil, but remained evident in the deeper layers. Biochemical indicators of soil organic matter suggested that biochemical recalcitrance of soil organic matter was higher under pine than under oak, contributing as well to a greater SOC storage under pine. Differences in SOC stocks between tree species were mainly due to the particulate organic matter (not associated to mineral particles). Forest conversion from Pyrenean oak to Scots pine may contribute to enhance soil C sequestration, but only in form of mineral-unprotected soil organic matter. © 2011 Elsevier B.V.