Soils and vegetation are the second largest global carbon reservoir. Carbon storage in soil can be increased by increasing carbon inputs and/or reducing decomposition rates. Reduced tillage practices have the potential to increase soil carbon storage by reducing decomposition of soil organic matter and/or crop residue. Isotope values (\del) of soil respiration can help infer the contribution of soil carbon pools to the soil CO2 flux, providing insight into the effects of tillage on residue decomposition. The objectives of this paper were to: (1) measure the isotope signatures of respired CO2 from conventional tillage and no-till plots, (2) compare and contrast the two treatments in the fall and in the following spring before planting, and (3) compare the \delta^{13}C of soil respiration of the experimental plots with the surrounding region. Isotope soil CO2 fluxes were measured using tunable diode laser spectroscopy and micrometeorological methods in a field experiment in Ontario, Canada. Measurements were made in fall following corn (a C4 plant) harvest and again in spring prior to soybean emergence. Data were used to compute the \del value of respired CO2 using both the flux ratio and Keeling plot methods. The large concentration footprint prevented the Keeling plots from discerning the treatment effect. In the conventional till plots, the respired \del value showed a stronger C4 signature (-16.7\pm2.5\permil)compared to the no-till field (-20.2 \pm2.7\permil) which had no soil incorporation of above-ground crop residue. This indicates more rapid decomposition of the `new' residue in the conventional tillage treatment. Both treatments showed a decrease in the isotope ratio during the spring measurements (-20.6\pm3.7\permil and -24.2\pm3.\permil for the conventional and no-till plots, respectively) which shows a depletion of the labile C4 substrate and a shift in respired substrate towards the soil C3 organic matter over the fall to spring period. For the fall, we estimate that 57 and 25% of the CO2 flux originated from crop residue for the conventional and no-till systems, while in the spring the proportions had decreased to 22 and 0%, respectively.