| Summary: | Acid sulfate soil (ASS) has major problems related to phosphorus deficiency and high potential for N<sub>2</sub>O 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 <i>in vitro</i> incubation studies. An ASS was amended with two types of rice husk biochar (at rates of 0 g kg<sup>−1</sup>, 20 g kg<sup>−1</sup>, and 50 g kg<sup>−1</sup>, equivalent to 0 Mg ha<sup>−1</sup>, 20 Mg ha<sup>−1</sup>, and 50 Mg ha<sup>−1</sup>, assuming that bulk density was 1 g cm<sup>−3</sup> and evenly applied for 10 cm in depth) and compost (at rates of 0 g kg<sup>−1</sup>, 10 g kg<sup>−1</sup>, and 20 g kg<sup>−1</sup>, equivalent to 0 Mg ha<sup>−1</sup>, 10 Mg ha<sup>−1</sup>, and 20 Mg ha<sup>−1</sup>) and incubated. Application of compost increased labile P by 100% and 200% at rates of 10 g kg<sup>−1</sup> and 20 g kg<sup>−1</sup>, respectively. Both biochars showed an increase in NaHCO<sub>3</sub>-soluble inorganic P by 16% to 30%, decreases in NaOH-soluble inorganic P and NaHCO<sub>3</sub>-soluble organic P. N<sub>2</sub>O emissions were significantly decreased by 80% by a biochar with a higher surface area and higher NH<sub>4</sub><sup>+</sup> adsorption capacity at a rate of 50 g kg<sup>−1</sup> as compared with those in un-amended soil. In contrast, compost amendment at a rate of 10 g kg<sup>−1</sup> significantly increased N<sub>2</sub>O 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.
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