Acid sulfate soil (ASS) has major problems related to phosphorus deficiency and high potential for N₂O emissions, as well as strong acidity. The objective of this study was to evaluate the effects of rice husk biochar and compost on P availability and greenhouse gas (GHG) emissions in ASS in in vitro incubation studies. An ASS was amended with two types of rice husk biochar (at rates of 0 g kg⁻¹, 20 g kg⁻¹, and 50 g kg⁻¹, equivalent to 0 Mg ha⁻¹, 20 Mg ha⁻¹, and 50 Mg ha⁻¹, assuming that bulk density was 1 g cm⁻³ and evenly applied for 10 cm in depth) and compost (at rates of 0 g kg⁻¹, 10 g kg⁻¹, and 20 g kg⁻¹, equivalent to 0 Mg ha⁻¹, 10 Mg ha⁻¹, and 20 Mg ha⁻¹) and incubated. Application of compost increased labile P by 100% and 200% at rates of 10 g kg⁻¹ and 20 g kg⁻¹, respectively. Both biochars showed an increase in NaHCO₃-soluble inorganic P by 16% to 30%, decreases in NaOH-soluble inorganic P and NaHCO₃-soluble organic P. N₂O emissions were significantly decreased by 80% by a biochar with a higher surface area and higher NH₄⁺ adsorption capacity at a rate of 50 g kg⁻¹ as compared with those in un-amended soil. In contrast, compost amendment at a rate of 10 g kg⁻¹ significantly increased N₂O emission by 150%. These results suggest that in ASS, whilst compost is more effective in improving P availability, biochar is more effective in mitigating GHG emissions, emphasizing that fundamental characteristics of organic amendments influenced the outcomes in terms of desirable effects.