Agroforestry systems, i.e. agroecosystems in which trees are intercropped with annual crops or with grasslands, have the capacity to store more carbon in the soil in comparison to reference systems [1, 2, 3]. The residence time of the carbon additionally stored in the soil is critical in the perspective of mitigating greenhouse gas emissions. To our knowledge, the temporal stability this carbon has not been assessed yet. To do so, we used a long term agroforestry experiment located in southeast France, where hybrid walnuts have been planted 18 years ago, with a density of 110 trees ha-1 and are intercropped with durum wheat. The soil is a silty and carbonated deep alluvial Fluvisol. Soil OC stocks have been measured on this site and we sampled the tree row, the inter-row and the reference plot down to 180 cm. Samples have been incubated in the laboratory at pF 2.5. The evolved CO2 was monitored as well as its ɷ13C. The measurements were performed in order to quantify the contribution of soil organic matter and carbonates to the evolved CO2. We found that carbonates made an important contribution to the evolved CO2, with up to 70% of evolved CO2 presumably originating from the carbonates. There was no different of SOC mineralization rates between the tree row, inter-row and control plot except in the 0-10 cm layer where it was larger in the former. This could be explained by the abundance of SOC in this layer of tree rows corresponding to particulate organic matter. The mineralization rates of SOC decreased with depth, showing the increased stability of SOC in the subsoil. We found no evidence of increased stability of the organic carbon stored in agroforestry plots. Most of the additionally stored organic C occurs in the surface layer and is labile (particulate organic matter). Below 10 cm rates of mineralization as measured in vitro are similar, but more SOC is present in the agroforestry plot, in particular in the tree row. This study also shows the importance of accounting for inorganic carbon when measuring mineralization rates of organic matter in calcareous soils.