There have been increasing interests in the conversion of organic residues into biochars in order to reduce the rate of decomposition, thereby enhancing carbon (C) sequestration in soils. However energy is required to initiate the pyrolysis process during biochar production which can also lead to the release of greenhouse gasses. Alternative methods can be used to stabilize C in composts and other organic residues without impacting their quality. The objectives of this study include: (i) to compare the rate of decomposition among various organic amendments and (ii) to examine the effect of clay materials on the stabilization of C in organic amendments. The decomposition of a number of organic amendments (composts and biochars) was examined by monitoring the release of carbon-dioxide using respiration experiments. The results indicated that the rate of decomposition as measured by half life (t1/2) varied between the organic amendments and was higher in sandy soil than in clay soil. The half life value ranged from 139days in the sandy soil and 187days in the clay soil for poultry manure compost to 9989days for green waste biochar. Addition of clay materials to compost decreased the rate of decomposition, thereby increasing the stabilization of C. The half life value for poultry manure compost increased from 139days to 620, 806 and 474days with the addition of goethite, gibbsite and allophane, respectively. The increase in the stabilization of C with the addition of clay materials may be attributed to the immobilization of C, thereby preventing it from microbial decomposition. Stabilization of C in compost using clay materials did not impact negatively the value of composts in improving soil quality as measured by potentially mineralizable nitrogen and microbial biomass carbon in soil. Stabilization of compost using clay materials (e.g. allophane) enhances carbon sequestration in soils. Display omitted ► Comparison of decomposition rate between composts and biochars. ► Heavy metals influence the decomposition of composts in soils. ► A novel approach using clay materials to enhance carbon stabilization in composts. ► The C stabilized composts enhance soil quality and add to the long-term soil C pool.