Abstract Precise methods for the detection of geologically stored CO 2 within and above soil surfaces are an important component of the development of carbon capture and storage (CCS) under terrestrial environments. Although CO 2 leaks are not expected in well-chosen and operated storage sites, monitoring is required by legislation and any leakage needs to be quantified under the EU Emissions Trading Directive. The objective of the present research was to test if 13 C stable isotope motoring of soil and canopy atmosphere CO 2 increases our detection sensitivity for CCS-CO 2 as compared with concentration monitoring only. A CO 2 injection experiment was designed to create a horizontal CO 2 gradient across 6 m × 3 m plots, which were sown with oats in 2011 and 2012. Injected CO 2 was methane derived and had an isotopic signature of −46.2‰. The CO 2 concentrations were measured within the soil profile with passive samplers and at several heights within the crop canopies. The CO 2 fluxes and their 13 C signatures were also measured across the experimental plots. In situ monitoring and gas samples measurements were conducted with a cavity ring down spectrometer (CRDS). The plots displayed hot spots of injected-CO 2 leakage clearly detectable by either concentration or isotopic signature measurements. In addition, the 13 C signature measurements allow us to detect injected CO 2 in plot regions where its presence could not be unequivocally ascertained based on concentration measurement alone.