Recent studies have suggested that belowground C sources in agro-ecosystems are of greater importance than aboveground sources with respect to SOC accumulation. The aim of this study is to determine the effects of management on the magnitude and timing of root residue contribution to soil respiration. Use of ¹³C isotope labeled root residue and static chambers allows us to determine this contribution among 5 agricultural management practices common to the Upper Midwest. These systems vary in tillage type and frequency, rotation length, and the form and quantity of fertilization. The ¹³C loss as CO₂ is tracked over a growing season and is used to separate root residue contribution to total soil respiration from other SOM sources. Preliminary results indicate that the contribution of the labeled root residue to the total CO₂ respiration declines across the season among all management practices, but the timing of this contribution varies. The variability due to spatial location, primarily topography and correlated soil physical properties, may be an equal or perhaps greater source of variation than management effects. With these results we will be able to determine the relative importance of management and spatial variability on SOM dynamics over a growing season.