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 139 days in the sandy soil and
187 days in the clay soil for poultry manure compost to 9989 days 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 139 days to 620, 806 and 474 days 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.