Beyaert, R. P. and Voroney, R. P. 2011. Estimation of decay constants for crops residues measured over 15 years in conventional and reduced tillage systems in a coarse-textured soil in southern Ontario. Can. J. Soil Sci. 91: 985-995. Crop residues are the primary means of sustaining soil organic matter levels in agricultural soils. This study was undertaken to determine the effects of tillage practices on the rate of decomposition of crop residues over a 15-yr period under field conditions in southern Ontario. Microplots were amended with ¹⁴C-labelled above-ground residues of five annual agricultural crops: corn (Zea mays L.), soybean (Glycine max L.), winter wheat (Triticum aestivum L.), winter rye (Secale cereale L.) and tobacco (Nicotiana tobaccum L.). The crop residues were added to the soil immediately following harvest during the 1990 growing season using a simulated conventional mouldboard plough-disc management (CT) or conservation tillage management (RT), and the amounts of crop residues remaining were measured periodically. The rate of decomposition of the labile C was positively correlated to the levels of hot-water soluble C and N content and negatively correlated to the C:N ratio and hemicellulose concentration of the residues. Decomposition of the residue C was greater under CT during the initial phase of decomposition, indicating that the incorporated residues were exposed to a more favourable environment for microbial activity compared with surface-applied residues. Kinetic analysis of residue decomposition showed that residues managed under CT had a larger labile component and faster rate of decomposition and a smaller resistant component with a slower decomposition rate than RT. Comparisons of models describing the decomposition of combined crops/tillage practices did not describe the decomposition process as well as models for individual crop/tillage combinations.