Subtropical recent alluvial soils are low in organic carbon (C). Thus, increasing organic C is a major challenge to sustain soil fertility. Biochar amendment could be an option as biochar is a C-rich pyrolyzed material, which is slowly decomposed in soil. We investigated C mineralization (CO2-C evolution) in two types of soils (recent and old alluvial soils) amended with two feedstocks (sugarcane bagasse and rice husk) (1%, weight/weight), as well as their biochars and aged biochars under a controlled environment (25 ± 2 °C) over 85 d. For the recent alluvial soil (charland soil), the highest absolute cumulative CO2-C evolution was observed in the sugarcane bagasse treatment (1 140 mg CO2-C kg-1 soil) followed by the rice husk treatment (1 090 mg CO2-C kg-1 soil); the lowest amount (150 mg CO2-C kg-1 soil) was observed in the aged rice husk biochar treatment. Similarly, for the old alluvial soil (farmland soil), the highest absolute cumulative CO2-C evolution (1 290 mg CO2-C kg-1 soil) was observed in the sugarcane bagasse treatment and then in the rice husk treatment (1 270 mg CO2-C kg-1 soil); the lowest amount (200 mg CO2-C kg-1 soil) was in the aged rice husk biochar treatment. Aged sugarcane bagasse and rice husk biochar treatments reduced absolute cumulative CO2-C evolution by 10% and 36%, respectively, compared with unamended recent alluvial soil, and by 10% and 18%, respectively, compared with unamended old alluvial soil. Both absolute and normalized C mineralization were similar between the sugarcane bagasse and rice husk treatments, between the biochar treatments, and between the aged biochar treatments. In both soils, the feedstock treatments resulted in the highest cumulative CO2-C evolution, followed by the biochar treatments and then the aged biochar treatments. The absolute and normalized CO2-C evolution and the mineralization rate constant of the stable C pool (Ks) were lower in the recent alluvial soil compared with those in the old alluvial soil. The biochars and aged biochars had a negative priming effect in both soils, but the effect was more prominent in the recent alluvial soil. These results would have good implications for improving organic matter content in organic C-poor alluvial soils.