Soil plays a major role in the carbon cycle for both carbon stocks and respiration since soil organic carbon (SOC) is the most important carbon pool and soil CO 2 efflux is the largest source of carbon emissions to the atmosphere. The aim of this study was to understand how the combination of no-till (NT) and cover cropping (CC) influences both SOC and soil respiration. Soil respiration was measured in southwestern France from April to October at two distinct sites characterized by contrasting initial soil carbon contents. Each site included two adjoining maize fields in which conservation (NT+CC) and conventional (CT) practices were used. For soil with initial high SOC contents (> 50 tC ha-1 in the 0-30cm layer), SOC stocks at the 0-90 cm depth (NT+CC: 72.4 ± 10.6, CT: 68.9 ± 9.2 t C ha-1) and respiration during the maize growing season (NT+CC: 0.57 ± 0.24, CT: 0.54 ± 0.26 g CO 2 m-2 h-1) were not significantly different for both practices. For soil with initial low SOC contents, SOC stocks at the 0-90 cm depth (NT+CC: 42.7 ± 5.5, CT: 29.7 ± 4.8 t C ha-1) and respiration (NT+CC: 0.86 ± 0.32, CT: 0.50 ± 0.34 g CO 2 m-2 h-1) were higher with the NT+CC practices than with the CT practices. Due to higher cover crop biomass and root density, SOC mineralization and root respiration were enhanced. We showed a positive effect of conservation practices on the SOC stock in the upper layers but also in the deeper layers (60-90 cm) only in a low initial carbon content environment. At high SOC levels, the soil may have reached its carbon sequestration potential, or the time since practices were implemented was not long enough to observe the effects. Our study shows that the potential of NT+CC to sequester large amounts of SOC, enabling the mitigation of greenhouse gas emissions, strongly depends on initial SOC contents.