Establishment of groundcovers (GC) is an efficient practice to reduce soil and nutrient losses in olive orchards, so they can act as a sink of atmospheric carbon and improve soil fertility. The aim of this study was to assess the carbon sequestration potential of several species used as groundcovers in two olive orchards. The experiment was conducted during three growing seasons in two olive orchards in Andalusia (Spain). In an experimental field, a grass (Brachypodium distachyon) and two crucifers (Eruca vesicaria and Sinapis alba) were used; in the other experimental field, three legumes were sown: common vetch (Vicia sativa), bitter vetch (Vicia ervilia) and hairy vetch (Vicia villosa). In both fields the species were sown and compared with the spontaneous vegetation of the area. The carbon release from groundcovers was studied and soil organic carbon (SOC) analysed during the decomposition period to assess the atmospheric carbon fixation. The increments of SOC in the first 20 cm of soil reached higher values with crucifers and grass than legumes. Sinapis alba obtained the best result with 2.56 Mg SOC ha−1 yr−1. Establishment of groundcovers are an efficient tool for atmospheric carbon sequestration and to protect the soil from erosion.