Researchers use stable isotopes to measure directly the rate at which litter decays and to trace the flows of its C within soil pools over the long term. Barley straw enriched with ¹³C (~10 atom%) was applied at 10 Canadian sites in 2007 to determine how soil type, climate, and residue placement affect the long-term fate of this residue in soil organic matter. Soil samples were removed at about 0, 0.5, 1.0, 2.0, and 3.0 yr after residue application and analyzed for ¹³C enrichment to estimate decomposition rate in the early phases of the experiment. Across all sites about 70% of the residue applied in fall 2007 decayed and was mineralized and lost as CO₂ after 3 yr. The recovery of applied ¹³C declined sharply at first, with about 25% lost during the initial over-winter period (½ yr). This was followed by another 25% loss during the first growing season (second ½ yr), and another 25% during the following 2 yr. Cumulative soil temperature (measured at 5 cm deep over the 3-yr period) accounted for much of the difference in residue decay at most of the sites. Differences in C mineralization were observed between incorporated and surface-applied residue treatments within the first year. Initially, surface-applied residues appeared to decompose more slowly than those incorporated into the soil, but only at the drier sites, and this placement effect diminished with time. These early findings, though still preliminary, substantiate the usefulness of isotope-based experiments in understanding residue decay. A future aim is to incorporate this understanding into improved simulation models.