Organic substrate input stimulates mineralisation of native soil organic matter, resulting in CO2 priming. Our previous studies showed that such CO2 priming enhanced alkalinity release but the mechanisms behind this are unknown. This study used 14C-labelled compounds to investigate the effect of added organic compounds on decomposition of soil organic matter and how this related to the enhanced release of alkalinity. 14C-labelled glucose and malic acid were added at a rate of 1 mg C g−1 to topsoil and subsoil of the Kandosol (pH 5.4–5.8, C 8.9–12.4 mg g−1), the Podosol (pH 4.4–4.5, C 1.5–2.9 mg g−1) and the Tenosol (pH 4.7–6.1, C 1.9–10.9 mg g−1), and incubated for 15 d. 21–27% of the added C was mineralised to CO2 in the Podosol while 56–74% was mineralised in other two soils with malic acid being mineralised more than glucose. The CO2 priming, as a result of added C, was substantial, and ranged 110–325 μg g−1 for Podosol and 766–1178 μg g−1 for the other two soils with the priming being greater in topsoil than subsoil. The addition of both organic compounds resulted in alkalinity priming in the Kandosol and the Tenosol but not in the Podosol; the alkalinity was greater with malic acid than glucose and greater in topsoil than subsoil. The effect of glucose on alkalinity release occurred mainly via NO3 immobilization while the effect of malic acid via ammonification, NO3 immobilization and decarboxylation/decomposition of native soil organic matter. This study confirmed that alkalinity priming occurred with concurrent CO2 priming as a result of C compound addition. This alkalinity priming depended on added C source, initial soil pH and soil organic matter content.