In the present study, three soils (Ferralsol, Luvisol topsoil, and Luvisol subsoil) were amended with biochars (charcoal, hydrothermal carbonization coal (HTC) of bark, and low-temperature conversion coal of sewage sludge), wheat straw and a control (no amendment) and incubated over a period of 365 days. Each amendment was applied at a rate of 11.29 g C/kg soil. After incubation, the soils were analyzed to retrieve three density fractions (free fraction (FF), intra-aggregate fraction (IAF), and heavy fraction) which were analyzed for total carbon (TC) contents and scanned by fourier transform infrared spectroscopy (FTIR). The biochars and straw significantly increased the TC contents of soils as compared to control. Among soil organic matter (SOM) density fractions, higher TC contents were documented in the FF and IAF from biochar treatments as compared to the straw. The FTIR spectra of the FF from the charcoal and HTC treatments showed the presence of aluminosilicate minerals on surfaces of SOM. There were slight changes in the FF of straw and HTC treatments as compared to spectra of original amendments. The study suggests that the stability of charcoal and HTC in soils is due to the recalcitrant nature of biochar followed by occlusion into soil micro-aggregates.